Leaving a Master's Degree Early for a Doctorate Degree?
If you're in your last semester, the best path forward is to finish your MA thesis. Your dissertation does not need to follow directly from it, particularly if you're studying in the social sciences.

Finish the thesis for a few reasons. First, finish it for the MA degree. You've (presumably) paid for 3 semesters of coursework. Failing to finish would mean that, essentially, you've wasted the past three semesters. Second, finish the thesis for yourself. In 20 years, you won't care a whit about your strained relationship with your mentor, but you will care that you saw the program through, even when the going got tough. Third, finish the thesis so that you don't have to explain it in the future. People will wonder why you failed to finish your thesis, and it might cast doubt on your ability to stick through tough/challenging projects. Academia is a small place, and you don't want to be known as the guy who didn't finish his master's degree.