I'm not even sure the kid is bad per se (although obviously his behaviour is) but loads of people I know say they find 4/5 yo old boys to be tremendously challenging and often a downright pain in the arse. The testosterone comes in and suddenly previously sweet little boys can decide that, say, spending all evening kicking mummy in the shins after she gets back from work is the most tremendous fun and that can be a tad overwhelming.
they are. or, at least a lot of them are
I want to contrast 3 different experiences of mine that I think show the spectrum, or part of it anyway.
1. I was a nanny to a 4 yr old (had just turned 4)
a very sweet kid from a well-adjusted family. He was on the quiet, sensitive side, but still loved crashy-smashy games most of all, was very rambunctious and unaware of his strength or ability to hurt people, and I definitely went home with bruises all over me pretty much every day

His little feet were always kicking, and he couldn't sit still.
2. a 4 yr old boy from a more chaotic family...mom and dad were recently divorced, the house was a mess, and the kids were well loved but the mother admitted she did not have the best parenting skills. He was like the other 4 yr old with the preference for violent and agressive play, but he was more of a problem in that he lashed out at others a lot, and seemed to have a harder time understanding why he should care if he accidentally hurt someone during play or just in general. He also used to grumble and talk a lot about blowing or cutting people's heads off

if they did something he didn't like. He talked back, had frequent tantrums, and had a very poor sense of rules or things he should or shouldn't do. Kind of a "wild" child.
I worked on curbing the agressive behaviors with both of these kids, for their own good as well as the fact that I couldn't deal with being a punching bag every day at work. I did research and asked people I knew who worked in child counseling etc for advice. It worked, but also they both had wonderful personalities once you got past the rough spots. They were both very silly and sweet. You learn that most of all they just need lots of love and guidance, and for all the tough guy act they really are very little and vulnerable.
3. a boy of 6 that I watched for 2 yrs. He's a good example of what can happen if you don't take any action to deal with aggression when they're younger. At this point the boy was very big for his age (wearing size 10 or 12 boys - here kids sizes correspond to age) so sort of a little boy stuck in a big boy's body he wasn't sure how to control yet...very intelligent, but scarily violent, aggressive, and verbally abusive (saying I hate you & other nasty comments to everyone if he didn't get what he wanted). He was too rough with his sister and other kids, and most play time ended up with someone crying because of an injury he'd caused, whether accidentally or on purpose.
I really struggled with this kid, and was ready to give up several times. I remember thinking he was a devil's spawn (in fact, I think I thought that about kid 2 also, which is why I can relate to Yelkcub's reaction but why I also know it's wrong).
His parents were divorced too, and the boy actually "caused" his mother and her boyfriend to break up because the boyfriend also had two sons, both of whom got badly injured by the boy on several occasions and he (the dad) became very frustrated with the whole scene and just couldn't take it anymore.
At that point she started the boy in counseling, and I had also started to work with him in similar ways to the younger ones, and I think we all just made a real effort to help him get better control of himself.
Again, he was a sweet boy underneath it all. Very intelligent, kind and thoughtful when he wanted to be, creative, and empathetic.
so, having these experiences, I can't believe that any child is bad, or that this type of thing is uncommon, or that they're actually trying to hurt people when they do this. Part of it is biological, part of it is environment, and all of it is manageable (well most of it anyway...you do also just have to let them be boys/ kids to some degree and back off)