Depends on so much.
Wonder Woman killing Maxwell Lord is very different to the Red Hood slaughtering low level punks. She used the lassoo of truth to get him to admit that the only way to stop the mind control on Superman was to kill him. There was no other option, he had to die, and he had to die then.
Captain America killed during World War II, and should have done, because that is part of war. Punisher is an anti-hero, Wolverine is THE anti-hero from which all other comic book anti-heroes spring. Hawkeye apparently had a strictly no-kill policy for years which has only recently started to be re-written. Superman never kills, ever. Deadpool is an amoral psychopath and about half of them I can't find any record of them killing in the 616 or Earth 1 universe. when has Cyclops or Superman Doctor Strange killed?
I don't think heroes that kill are heroes, unless there are major extenuating cirucmstances, such as killing being the single recourse, or in a time of war. Heroes need to act within the remit of the law, and so their job is to bring criminals to justice, not to administer the justice themselves.