Dealing with potentially bad response from supervisor upon leaving with MSc for different PhD program To keep it brief, I am thinking heavily about leaving my current PhD program with a MSc after 4 years to embark on a PhD in a field I am more interested in, as I have grown to dislike my current field. There are a series of problems within my group, from a completely unaware and ambivalent supervisor, to internal issues, to a general lack of quality of life due to many factors. All of that bundled with my desire to pursue my passions has left me wanting to move on to a different school.

I am, however, worried about my supervisors reaction if I ask him to do so. I know from past experience he has been pretty hostile to students who have wanted to do this, and furthermore, if he gives me a reference letter, I don't know how good it will be. I know I am a good student with good technical and research skills, but he is so absent from the group here (as he directs another group overseas, where he spends 95% of his time) that I feel that he doesn't even know what I have been doing/am proficient in.

I am wondering what your advice is in dealing with this sort of situation; do I suffer through, and leave with a PhD in a field and topic I resent? Or do I go on to do what I love, at the risk of being extradited by my supervisor, potentially leaving me unable to do so? It really is a classical dilemma, and I am genuinely worried either way.

Thank you.
This is all opinion, so it is only worth what you pay for it, of course. But I would first consider how long it will take you to finish your current degree. It is pretty miserable being miserable for three more years, but if it can be done in a year then it might be best to just tough it out. 

There is no reason, in principle, why you can't change fields later, either gradually, from a paid position, or abruptly. If the fields are different enough you may need credentials there, of course. 

If you do complete the doctorate, then your old advisor's opinion about switching won't be of concern, only his/her opinion about the quality of your work. 

However, another alternative, if you have picked out a different university, is to quietly explore with them the possibility of switching. If you can find a compatible potential supervisor there, the current situation may not be a detriment to switching. Personal contact is probably best here as it enables you to make judgements about how much to reveal about the current dilemma. 

But if completion is a long way off, then quality of life would seem important enough to make a break, even if not all of the consequences are positive. But only you can balance the forces.