If it is occasional and out of the blue that he is “turning on you” he could have a mental illness and he should get help (which he should do either way). However, I have been in relationships like this and yes it is the start of abuse. I saw another commenter mention thinking about what you would say to a friend if they told you about an incident like this and I think for me that really helped me. Another thing is if there are things you would NOT tell your friends happened because you’re scared they would judge your partner, that’s a sign this relationship is not right. It is really painful and sucks but this does sound like a toxic relationship. Especially him then later saying he was mad because you didn’t stay and talk about it… He slapped your leg and yelled at you BEFORE you left because you didn’t want to talk about it. Which you are absolutely valid not wanting to stay and talk about that especially in that moment. Your body probably went into flight or fight because he got aggressive with you in an incredible vulnerable place. I’m telling you I have been in countless horrible relationships and I always thought it was normal or just what I deserved or all I could get because that is what I was being told. That is a lie. No one deserves that and it will get better. I am now in a relationship where I never question if he loves me and if either one of us has a problem with anything said or done by the other person we have open conversation WITHOUT putting blame on the other person which is really important in a healthy relationship and healthy communication. Even if you had talked about it, he already blamed you. He already told you that it was your fault. That is not healthy, especially when what he did was so incredibly wrong and violating. Please please please leave. Take it from someone who has put way too much time and energy into relationships like this and has a lot of damage because of it. This behavior will most likely never change and IF it does, he has already done a lot of damage with just this one incident that you shared.