A dealbreaker is typically final. It's not bolstered by attempts at getting the other person on your side by trying to scare them into giving you what you want, it's taking someone aside and saying, simply, for example, "I can't stay with a man who can't commit to me." You're not asking your partner to make a choice between keeping you or not; you're telling them that if certain conditions are met, you won't be happy and you'll have to move on.

An ultimatium, on the other hand, is more along the lines of, "Stop talking to other women or I'll leave you." It's one partner threatening the other in an attempt to ensure their own comfort and happiness. The person acting on a dealbreaker doesn't want either side of the relationship to have to compromise in uncomfortable ways, while the person giving ultimatums cares less about the other person's happiness and demands a compromise that their partner probably doesn't want to make.