I was dragging my child by the arm at the park because they were misbehaving
No way, was that really necessary?
I did it out of anger and I had misunderstood what the kid did
If you find yourself angry with at something that your child did, you should wait until you are not upset to punish your child. Waiting until you cool off will ensure that you do not harm your child by overreacting.
It might be too late, I'm taking care of the kid and the mother is dead.
I'm sorry for your loss, but that doesn't mean that you get to raise your child this way. It's not the kid's fault that you made a mistake, you aren't allowed to take it out on them. I believe you really need to try and improve yourself so your kid would have a better relationship with you.