Not sure the "why" really matters.
I never dated a psychopath (have you looked up the definition? Seems pretty scary) but I did waste 5 years of my life with a sociopath. The charm can pull you in and if you aren't strong enough it's very hard to make the right decision to get out.
None of us can make the choice for you to step away- only you can do that.
At least you are aware that this is a bad choice- you're ahead of the game in that aspect. Now it's time to make a choice- do you continue to make a toxic decision to spend time and energy on him (and risk your life?) or do you decide today that you are worth more, that you are stronger and that God would want much more for you than this.