Hi, I'm very sorry for what brings you here. But your ex is unstable alcoholic (which was most likely caused by his family). So I would cut off any contact with his mother.
On top of it, your ex knew how hard it is to live in abusive household and he didn't change anything for his 9 year old. He abandoned him and left him live with crazy mother and abusive man who ended up killing his mother. Of course that poor kid is angry! He was abandoned by his dad (who is unstable himself) and had to witness got knows what between his mom and her new man. That poor kid needs counseling and stable home.
It seems like if you stick with him, your kids might end up hurt and compensate for that by drugs and alcohol.
I strongly recommend therapy (it really helped me) and cheaper version this podcast which was tremendous help freedomain radio / stefan molyneux (also on YouTube section relationships is interesting).
I know it's hard not to have father for your children and that you feel guilty, but the man you described is not a father figure.
The best you can do is to apologize to your kids for your choices. And show them how healthy relationships look like.
I'm very sorry for what happened to you. I was in 2 yr relationship with an addict and I work really hard so this would not happen to me. However I guess there is no guarantee and I sympathiuwoth you.
Stay strong. He is no good.