Smoke and Illusions
From the first time Kurenai cast a genjutsu, it appealed to her.
Supposedly, that was because her whole clan was genjutsu oriented.
But Kurenai was something different. Kurenai liked to fool herself with her own genjutsu. She would slip out of the house at night and down to the creek a half-mile away and perform entire magic shows for imaginary audiences, using her genjutsu.
She especially liked dissolving away in a flurry of pink cherry blossom petals. It made her feel pretty.
As she grew older and got better with her illusions, the imaginary audiences weren't so imaginary anymore. After becoming chuunin, she incorporated them into her genjutsu act, giving them the faces of her fellow chuunin.
When she reached jounin, she added a stage and a few assistants out of trees.
She met Asuma when she was a jounin, and it was like love at first sight.
She had to cut back on the length of her illusionary life at night to make room for him.
Asuma was actually pretty close to normal up until he hit chuunin, Kurenai discovered (he still was pretty normal, compared to most; just look at Kurenai, she used her genjutsu on herself to become a magician).
His first ever mission as a chuunin, Asuma had to kill four men and watched his comrade die.
His cell leader offered him a cigarette and a bottle of sake that evening. The sake wasn't ever very appealing to him, but he never quit smoking after that night.
Because he believed the ghosts of everyone he ever killed were in the plumes of smoke he blew. So where most ninja drank away their ghosts, Asuma puffed his away, mumbling to them around the butt of his cigarette how sorry he was.
Sometimes Kurenai walked in on him in such a state, sagged into the couch, eyes screwed up staring into the smoke, muttering. The first time it happened, she wordlessly sat beside him and watched the smoke tendrils. The routine never changed.
Asuma knew what his insanity was doing to him, but he didn't really understand it was insanity until it was too late. He told Kurenai he thought he could feel his lungs blackening and falling apart with every toxin-laden drag he took. But he couldn't stop, because he was still going on missions and he was still killing people. When Ino and Chouji were gone, he added them.
So he suffocated on the rest of his smoky ghosts.
After that, Kurenai did away with her childish illusion. Instead, she wrapped herself in such a perfect illusion of her and Asuma that she couldn't remember it wasn't real.
No-one ever believed they were sane- Asuma apologizing to his cigarette smoke and Kurenai living in her own illusion. Except for that, they were pretty normal. I guess.
Kiba, Hinata, and Shino tried to free her, but she died in her imaginary world. Tsunade couldn't save her and Asuma's unborn baby.
Because you can't live on illusionary happiness.