Blended families are difficult under ideal circumstances. I would tread very lightly with suggesting that you kick the young man out. I don’t think you want to get into a “him or me” situation with your fiancé, because if the man is worth his salt, you will lose. Husbands/Wives come and go, but your child is yours for life.
I’m particularly disturbed by your comment that you hate this young man. Hate is quite a strong word, how would you feel if you discovered that your fiancé secretly hated your own child? I don’t know about you, but that would be a deal breaker for me.
One of the things I would focus on is setting up boundaries. I would discuss the fact that the son is disrespecting you verbally in your own home and that should not be tolerated whether he’s 18 months or 18 years.
Please remember that you are the adult in this situation, just because he’s 18 doesn’t mean he may feel that you have invaded his “space” (assuming you moved into the already established household of father and son). You brought a ten year old, younger kids get more attention. This young man may feel that your child may be replacing him in his father’s life.
Don’t engage, if he disrespects you, tell him in a calm voice that you will not be spoken to like that, if he continues ignore him. Ask your fiancé how he thinks you should deal with the situation when the boy acts up. I personally would not grant favors such as lending him my cell phone, if he is unable to act civilly.
I think the biggest mistake you can make is to compare this young man to anyone else. I have four kids; my youngest are identical twin 17 year olds. Each of them has their own strengths and weaknesses. Two of them have the exact same nature and nurture and they couldn’t be more different from one another.
If the boy genuinely has a drug problem, that should be the highest priority. Don’t forget addiction is an illness that has its own side effects, including behavioral issues.