Lara, psychology teaches us that we think the majority of people are like ourselves. We find it hard to believe some people are capable of what they ARE capable of. It is not a bad character trait, to trust others and to give them the benefit of the doubt.
I too, looked at my XH later and wonder how it was possible to fall in love with a man like that. Years later I appreciated the gifts he gave me. Thanks to him I became a much stronger person than I thought possible. I learnt so many lessons in how not to be from him.
Do not beat yourself up over this. His actions is not a bad reflection on you and can never be. At least you are not part of it anymore.