Venomosity

And then as an adult went "I'm going to apply this everyone except for myself. Everybody is worth it except for me." He extrapolates it out to everybody except for himself.

Quite literally the only thing that Rem really requests of him in the manga is for him to not take his own life lightly and she went into the "your ticket is always blank" talk to try and anecdote what she was trying to tell him: that there's always a light at the end of the tunnel and just because of one (granted very traumatizing and bad) thing he shouldn't allow himself to get sucked into the pits of despair - because she had been there.

Vash just goes "So everybody should get a second chance and all life is precious. I will never attempt to kill someone ever again." which really isn't what she said but that's how he extrapolates it out. Partially out of guilt because he feels responsible for the fall and everything that happened afterwards - like if he's said something or done something or stuck with Knives instead of allowing them to part ways then maybe none of what happened would have happened. He feels like he partially killed everyone who died in the crash. Like he's in part responsible for Rem's death.

If we go by that one piece of lore from trimax or tristamp (I forget where I heard it first), then plants were created from a sample of actual angels. Dogs were bred from wolves to be more obedient and helpful to humanity. In the same way, plants were bred from divinity to be more obedient and helpful to humanity.

From Knives' fear-tinged view, humans turned plants subservient as a perversion of their 'superior' wild nature. Humans put collars on angels and bred them to be amenable to lifelong servitude. He's a dog whose fear of humans has made him try to prop himself up as an untameable (unhurtable) wolf, not realizing that terrified obsession with humanity already sets him far apart from the real wolves.

Meanwhile, Vash started out from a different place. Unlike Knives, Vash originally had no strong desire to be loved by humanity. He was perfectly satisfied with Rem and Knives and only entertained passing curiosity about humanity, probably encouraged by Knives. When things go wrong and he lashes out and bites the hand that feeds, Vash gets to learn that he does care deeply for Rem. Maybe the instinct was bred into him by humans, but that doesn't change the fact that he hurt someone he cared about and that felt bad.

Through Vash's journey, he does seem on the surface more like the good dog who stays loyal no matter how much he's beaten down. But that's not quite right. Unlike Knives who runs away constantly out of fear, Vash spends time with people. He gets to know them. He learns about their unique, deep and varied lives. He doesn't care for people because he is beholden to his nature, he cares because he knows there's something to be cared for.

Vash learns that dogs didn't become dogs because humans forced them into the shape of the perfect slave - dogs became dogs because they were loved and to be loved is to be changed.

ARCANE WAS INSPIRED BY TRIGUN??