Ahh yes. This is a common theme on this board. What came first, the chicken or the egg?
It does not matter. The behavior and actions, all that matter, are what they are. The reason why just does not matter.
I hope you spend less time trying to focus on why, and more time getting support for YOU, and that poor child he did that to.
Hugs. I am sorry for what brings you here, but glad you are here.