Harley Quinn and Deadpool are probably two of the craziest and deadliest killers around. They may be from different universes, DC and Marvel, but if the two did end up having kids, I certainly wouldn’t want to be the one looking after them.
Venom - created by Todd McFarlane, Eddie Brock wheres the former symbiote costume of Spider-man becoming the greatest nemesis Spider-man has to face. I used to own a beat up Amazing Spider-Man #300 where he made his first full appearance. He was a favorite character to follow in Marvel Comics.