From at least the 1960s, Marvel's heroes were very different from their distinguished competition's. The main difference is that Marvel's heroes were more human than all-powerful icons like Superman and Wonder Woman. However, killing was verboten for both of these very different kinds of heroes. From the start, almost every Marvel hero believed that killing was completely wrong, even those who seem like they would be killers.
Source：https://www.cbr.com/why-superheroes-marvel-comics-wont-kill-spiderman-iron-man/
