A/N I just wanted to try and work out why Bellamy is working with Pike because it seems strange that he'd be so prepared to murder Grounders led by Indra that are protecting them so I just tried to get into his head and see what might have made him think it was the best thing to do.
Maybe it began when his mother got pregnant with Octavia, his disrespect for authority that is. He was taught from the moment he began to look at the world that the words of family and threats were the only key ones, people he could trust and love and do anything for, or people who would never be more than obstacles and a constant source of paranoia. He was strong for his mother and Octavia till his strength wasn't enough. He was strong till they tore his family apart, executing his mother and arresting Octavia. Then he was strong again and got himself to the place where he could be with his little sister again. Bellamy didn't have anything else but the knowledge that family was all that mattered and that sometimes violence was the only way to keep them together and whole.
("Wrong" whispers a fragment of his heart that still beats. "Clarke and Finn proved that peace could be achieved through sacrifice and hope. Genocide isn't the answer.")
Since they arrived on the Earth, the Grounders have systematically taken everything from him – his self-worth, his sister, his Clarke, the people he swore to protect. He can't forget that it wasn't many months ago that they were hunted and murdered, despite that being avoided even in the Ark. That he killed 300 people out of sheer idiocy and fear. He can't ignore that Lexa's betrayal led to Mount Weather, and he doesn't forget that there are now 39 more lives he lost to Mount Weather and Grounder betrayal. Gina deserved better, more than this. More than his broken heart. More than the monster he's become (and maybe always was) and around her sometimes, if he made an effort, he could hope one day to live again. They have ripped that from him too. He also remembers every second of those three months that Clarke ran. Left him. Alone. The girl who told him he wasn't a monster. The girl who had the sun in her hair and a desperate darkness mimicking his. She left and he had to deal with his mounting, monstrous kill count alone – she had to leave he understands that but anger and hurt bubbles deep; hardening to a distrust and near hatred of the Grounders that led them to that precipice. More than that Clarke feels lost to him. She's becoming one of them as a way to cope and his skin crawls with revulsion. He can't see her be one with the people that ripped them apart. (He also can't function knowing that she chose to stay with them when he was begging her to come home with him).
His trust of the Grounders was fragile at best and led by Clarke's belief, but every hope they ever lit was stamped out with a vindictive relish during every betrayal and murder of his own. Most of those graves are his burden to bear too, but he can hate the Grounders almost as much as he hates himself for them. He is alone because no one needs him and who is he without someone to protect?
Maybe he crossed an irreversible line when he was prepared to shoot the Chancellor to save his sister or maybe he crossed it later but all he knows is that it's been crossed. What more can he do? Is it a surprise that Pike's seductive words of protecting those closest to him work to convince him? That Pike manipulates his hurt and pain over Mount Weather. He's desperate and alone and finally, finally someone says he's needed like he was when he had a home. The Ark taught him that crimes are punished with almost excessive force, so when insecure he relies on the two facts of his existence – the threat of any and all persons betraying his family and the desperate need to prevent that. Peace and second chances are foreign concepts down here as they were in the Ark.
So when Bellamy cleans his gun, looks at his armour in the mirror and turns to leave, he feels like maybe it's the wrong thing to do again but down here the only choice is putting your family first.
(He's hollow and so past caring, that he barely notices his heart shattering what little was put back together when he saw Clarke and felt hope.)