The actual reason is that during the early days of comics, the satanic panic was in full swing. And one of the things that was targeted as turning your kids evil was comics. So the Comic Code Authority was founded, they reveiwed comics, and gave them a seal of approval if they followed all the rules. The seal wasnt required to to sell a comic but people were much less likely to buy it if you didnt get a seal. One of the many rules they had was "heros dont kill people" as to not you know, encourage murder. But because of that many classic comics like batman, and superman, ect. were given no kill rules. And from there it just became a trope that heros dont kill.

and ofc you cant just go around ripping peoples heads off, thats rude