I am really glad you wrote.
What's going on here is that you fiance is a drug addict andis fully engaging in addict behavior. The rages are (1) to stop you from interfering in his drug use and let him do whatever he wants and (2) brain damage from pot.
He is not going to change anything until he loses it all; his home, his family, his job, his dignity, his self-respect - absolutely everything.
It sounds trite, but it's actually a very real issue that addicts struggle with - and it's called denial. Denial is when someone truely has no sense or understanding of what is REALLY happening. As far as your fiance; he has no idea how bad it all is - he thinks his realy problem is everyone else.
For you to gain a real sense of what Denial is, read the "Alcoholics Anonymous Big Book"; it tells stories of how people experience denial- written by real addicts. The drug, whether it's pot or alcohol or gambling makes no difference - addiction is addiction is addiction.
There is NOTHING you can do to impact this process. We used to think that with adults, we could take things away, give them other things, maneuver their addiction and behavior for them, but what we know now, is the addict needs for it to all fall apart and any attemtps we make to help that along, really don't help. People around the addict just need to walk away to save themselves.
This is a sad and heartbreaking disease. What you need to do is understand and conceptualize that his disease is nothing you can impact and you need to view it as kidney failure, or brain damage, or diabetes, or any other disease that you cannot control; you have to understand that this is his problem and only he can fix it. If he had kidney failure (just as an example) you could purchase back his stereo to impact the disease - it's the same here - nothing YOU do will help him. BUT he can help himself.
For him that help will come with AA. Not NA, they are not as strong - AA has the best recovery anyone can find. But, he's not even going to look for help until his life hits rock bottom - it just doesn't happen any other way. Again, it's terribly sad, but that is the way it happens for addicts.