Could you be with someone who was causing your child to self harm?

Me and my son are living with my mom right now. We just moved out of my ex fiancé house. I found out that my son had been self harming. One of his friends told me and when I told told him to tell me the truth he denied it, until I made him show me. When I asked him why he said it's because my fiancé (at the time) made him feel so bad about himself. About his interests and made him hate himself. I only remember one thing my fiancé said to him. He called him a fagot for doing his girlfriend's make up, but apparently he had been saying a lot of stuff to him that I didn't know about. When my fiancé came home from work I automatically jumped down his throat about the things he said. He told me my son was being a pussy and he's just doing it for attention. I packed up our things and left to my moms. He started messaging me saying he loved me and wanted me to come back. That we have a wedding less than two months away. He then started demanding I pay him the money back from stuff he has spent on the wedding. Then he followed that with an apology that he shouldn't have gone off on me. He started texting my son and told him whatever guy I'm fucking can have his sloppy seconds. Who tf sends that to a teenage boy?! Then followed by another apology to me saying he will change and even go to therapy and we can do premarital counseling. Fir me personally, if you've pushed my son far enough to self harm, you're dead to me. I should have left sooner... I feel like I was blinded by the red flags. I thought his behavior was so sudden, but looking back he had been leaving red flags all over the place. I hate myself for my child having to go as far as cutting for me to realize it and I'm a shit mom for that. I'm gonna do everything I can to make it up to him.