Heroes are, unfortunately, not necessarily trained in restraint after a certain point, so even if it isn't in their plans, it can easily happen that either they destroy a villain, or an innocent bystander.
I always play it that Heroes are not allowed to kill unless they exhaust all options, but even then they are susceptible to the consequences of killing someone, no matter who it was.
I think, more than being capable of murder though, they're more apt to actually beat someone to within an inch of their life. Kusakabe will canonically fucking fight people even if he's severely outweighed. Same with Taemin.
and if I bring up someone that I haven't made a journal for yet, The Kid is also canonically a killer of men. He doesn't necessarily revel in it, but he's ready to just pop someone for inconveniencing him without a second thought.