Orochimaru
People don't matter to Orochimaru. When you're caught up in the quest for immortality, few things do matter, and if you're going to be cutting people open or subjecting them to gruesome experiments, then eventually it becomes imperative that you think of them, not as people, but as test subjects or guinea pigs. After all, if you think of them as people, you just might start getting squeamish.
It wasn't always like this. Once upon a time, he had people who mattered to him. His parents. His teacher, Hiruzen Sarutobi. His teammates, Tsunade and Jiraiya. These were people he wanted to protect, and who he would have been willing to suffer for. In the case of Tsunade and Jiraiya, there was a time when he would have died for them.
That all fell by the wayside a long time ago. These days everyone he meets is just a means to an end, another stepping stone on the road to eternal life. People don't matter to Orochimaru.
Itachi
People don't matter to Itachi Uchiha. Ever since he was young, Itachi has been putting the good of the community over the good of the individual. Maintaining the society he lives in matters far more to him than any one person does. As a result the fact that the society he loves is made up of individuals, has in many ways, become lost in the shuffle.
It's because of this dedication to the group that Itachi can massacre his entire clan despite his personal reservations. It's because of this dedication to the group that he can mentally violate his younger brother without considering the consequences of his actions. It's because of this dedication that he can assault former friends, kidnap Jinchuriki, and help Pain and Tobi on their insane quests, all in the name of preserving his cover.
The fact that he's done a lot of damage to his home country, driven his brother insane, and helped to contribute to the near end of the world is lost on Itachi. He did it because he was ordered to. He did it for the greater good. People don't matter to Itachi Uchiha.
Kisame
People don't matter to Kisame Hoshigaki. They don't matter because people lie. People deceive. People betray others, and are then betrayed in turn. If you cannot trust anybody, then in the end, how can you care about them?
Kisame learned this lesson the hard way, when he was forced to kill the cryptography unit he'd supposedly been assigned to guard. The looks of shock on their faces will haunt the swordsman for the rest of his life. It was at this moment that he realised that if he couldn't be trusted, than no one else could either.
Kisame wants a better world. He wants a world that doesn't produce people like him. In the meantime, he's willing to climb over mountains of corpses to get there. People don't matter to Kisame Hoshigaki.
Deidara
People don't matter to Deidara. That's because Deidara is an artiste, and the concerns of lowly peons are far beneath him. If they cannot appreciate his artistry, well, that's not his problem is it?
Ever since he was very young, Deidara has been obsessed with creating the perfect explosion. With creating that one, perfect work of art that, while seen only for a second, will last forever in the minds of those who witness it. That will scar their psyches so thoroughly that they will never be able to forget it.
He's willing to die for his art, and if that is the case, then why shouldn't others die for it as well? People don't matter to Deidara.
Sasori
People don't matter to Sasori of the Red Sand. This is because, in the end, other people just aren't that interesting to Sasori. Or even in a sense, entirely real. He's divorced from reality, living in a world where there's just him and his puppets. Other people are an intrusion on that world, and one that cannot be tolerated.
This inability to identify with others goes back a long way, to when he was a boy, using his puppets to take the place of the parents he had lost. Eventually it got to the point where the puppets, which he controlled and manipulated, were far more real to him then the actual people, who had minds and wills of their own. He can't really tell the difference anymore; in Sasori's mind, people are just automatons, shuffling around, a collection of parts whose desires don't register at all.
He's been making people into puppets for a long time now. When he's done with them, they're a part of his collection, and a part of his reality forever. He can see them and understand them now. They become real to him. People don't matter to Sasori of the Red Sand.
Kakuzu
People don't matter to Kakuzu. People aren't dependable. They won't watch your back, they won't pay your bills, and they won't keep a roof over your head. Money does all those things, and if a few people have to die to keep him in cash, well, that's just the cost of doing business, isn't it?
Once upon a time, Kakuzu believed in things besides money. That was before his village's leaders sent him on a suicide mission to kill the First Hokage. Before they arrested him and threw him in prison when he failed in that mission, but returned home. In that one moment, Kakuzu learned that people will ultimately let you down.
Money, on the other hand, never lets you down. It's real. It's dependable. You can never have enough of it, and if you have to cross some lines to get it that's entirely acceptable. Hurting people is fine, as long as it's in the name of your checkbook. People don't matter to Kakuzu.
Hidan
People don't matter to Hidan. They don't matter to his god either. Jashin doesn't want followers, he wants sacrifices. He wants pain, and misery, and bloodshed, and senseless violence, because those are the things that define humanity. With every person that Hidan hurts or kills, he spreads Jashin's message to the rest of the world.
That's the theory at least. In practise, for Hidan, the medium has long since become the message. He inflicts violence on others not to please his god, but because he, Hidan, likes violence. He lives for it, and there is no real way around it. Hidan enjoys fighting, and killing, and dragging others down to his own level. He's a teenager on an adrenaline rush, loving what he does, and doing what he loves.
Hidan revels in his own sadism. He tries to top himself with each new act of cruelty. He pleases himself, and if in the process he pleases Jashin, well that's fine too. There's plenty of victims to go around after all. People don't matter to Hidan.
Nagato
People don't matter to Nagato. He's seen too much and suffered too much. He's lost everything he ever cared about, and in the end, all that's left is a determination to make sure that this never happens to anybody again. He's dubbed himself Pain, because in the end, when he looks out at the world, Pain is all that he can see.
He can still see the Leaf Ninja who killed his parents. He can see his dog dying in front of him, collateral damage in a shelling. He can see Yahiko's face as the kunai slid into his chest, and feel his own shock, anger, and horror rising up inside of himself, until they overrode everything else. He's a shellshock victim, so badly traumatised that he can't see the positive side of anything anymore.
He's not proud of everything he's done, of the people he's hurt, or the trusts he's betrayed. What happened with Jiraiya is a wound that will never really go away. He'd do it again though. This world has to change. Individuals will suffer in the meantime, but when it's all over, they'll thank him for bettering their lives. People don't matter to Nagato.
Konan
People don't matter to Konan. Not anymore. That's because people don't matter to Nagato, and Konan thinks the way that Nagato thinks. She's been doing it for so long now that it's become second nature to her. So when Nagato starts ignoring the wants of others in his drive to better the world, Konan does the same.
It wasn't always like this. Konan used to have hopes and dreams of her own. There was a vision of the future that she, Nagato, and Yahiko all shared, and worked towards. It was a future she wishes she could have seen.
That future, and that Konan died with Yahiko. She's a shell now, doing what Nagato tells her to because she can't imagine doing anything else. The girl that she might have become is a distant memory. People don't matter to Konan.
Obito
People don't matter to Obito Uchiha. In fact he finds it downright offensive that they think they should matter. Rin is dead. The future he wanted is dead. And if he can't have what he wants, then why should anybody else?
People did matter to him at one point. Rin and Kakashi were his friends. They're not anymore. Kakashi's his enemy, and the real Rin, that he was crushing on? He barely remembers her. She's not a person anymore, just an idealised memory that he holds onto at all costs.
Obito isn't like Pain. He doesn't think that the world he's making will be better for everybody. It will be better for him though, and that's what's really important. His wants, his needs, his desires, these are the things that should be important to the rest of the world. Other people? Other people don't matter at all to Obito Uchiha.