katie,
I'm sorry you had to go through that. I do not have any experience with that level of behavior, maybe someone else will.
I just wanted to say, don't beat yourself up. Your son came to you with a legitimate request to do some work, you "paid" him in a way that he accepted - all seemed fine at the time - but then the drugs took over and things got out of hand. It's to be expected, but . . . there is no way you could have known it would happen. We make the best decision we can as each situation is presented to us.
Hope you have a better day today.
Joan