Would you be upset if a long-term partner confessed that s/he'd committed a serious crime before you met? How do you think it would affect your relationship?
Originally I would be hurt that they hadn't told me earlier in the "get to know one another" stage of our relationship. Most likely I would be surprised and sad because no one expects that their partner would be a criminal. Other factors would effect how I reacted to - what the crime is, if and how they were punished and their thoughts on that kind of behavior now. If it was a crime committed as a young adult and it wasn't "too serious" I'd feel more relaxed about it but if my partner had killed someone in their late twenties, that's a different story for me. I don't think that I would end a relationship over this but I might be wary of everything they had told me about their background.