"Un-alive" comes from a Marvel animated cartoon series called Ultimate Spider-Man, in which Spider-Man teams up with Deadpool, who expresses his intent to "un-alive" a certain villain. To which Spidey replies, shocked, "You mean KILL him?!"

Deadpool's circumlocution around killing and death is a parody of similar linguistic gymnastics from 1980s cartoons, which were considered "for children" and so addressing death directly was forbidden. And given that Deadpool's mental illness makes him genre-savvy, it was probably deliberate in-universe and out. The writers then paired that with Spidey using "kill" directly in an animated kids' block show, to show how ridiculous such censorship was.

The sheer irony is that we're now self-censoring to 1980s cartoon levels to avoid robotic censors we can't even argue with.