I sometimes wonder why some people care so much about their partner’s past, especially the not-so-good embarrassing past. The past literally means history, no matter what he or she did, right or wrong, it was his/her personal history. No history can be altered or changed.
He/She doesn't have to confess anything or any previous relationships, particularly not the details about how prior relationship happened and how it ended and who was the one to blame. He/She is not bound to let out all the truth.
If he/she does, it means, first, the person is honest, second he/she trusts the partner who listens, or third, he/she is absolutely stupid.
Then what do you want from him/her? If you can put up with the past, give him/her a chance; if you can't and you compulsively have to live within the past, then there is no need to torture yourself. Just pack up and leave.
Just from what he/she did and thought, which according to your judgement was not decent and morally correct, you are going to make an assumption that he/she has a dirty "soul"? It is too naive, judgmental, and idealistic.
Yes, you still have to take the risk that he/she may make some mistakes again, including the same mistakes because some humans can learn while some may have problems in correcting the awful behaviors due to psychological defects. That’s the risk you have to take with anyone. Nobody can guarantee you the one you are going to marry will never have change of heart and be absolutely loyal to you for the rest of his/her life. NOBODY. World is changing, people are changing, situations are changing... No one can say love will never change! We just do our best to maintain, to enjoy, and to hope for the happily-ever-after. Some can, some can't, in their entire lifetime.