Oh boy, I feel for you. To have a 5 year old doing this is crazy and so hard on you. This is a time when the boys normally are doing the exact opposite. Unfortunately, you don't give a lot of information.
It sounds like as early as 3 in preschool he was harming other kids? Wow! and you said there are no triggers? You have a stable marriage, no other kids, no birth problems? What tics him off? Is he sensitive to certain things that irritate him? How is he doing educationally in kindergarten? Is he at the level of the other kids? Does he have friends? What does he do at recess? When he gets home, what does he like to do? Does he talk with you or prefer to just go and watch tv or play video games? What made him mad in the classroom? I could go on and on.
Take a look at this post. it is a long one, but watch how the dynamics of the post change as more information is gotten and given. And some of this may apply to your child. https://www.medhelp.org/posts/Child-Behavior/8-year-old-step-son-extremely-sensitive/show/3017112
I can give you ways and books to help him control and express his anger. But I kind of need to figure out if it is anger or emotions, or anxiety that is driving him.
Do take the time to read the posts I mentioned, and get back to us with more information.