I think it is time to explain some boundaries to YOUR HUSBAND. Your son be danged, your hubby is the one I see as the bigger problem. I am sorry if this sounds disrespectful, but I would be leaving in a heartbeat if my son stood over my bed and my hubby took my son's side. I can honestly say this because I lived with a violent son, one who was determined to kill myself and my daughter. My son never saw my hubby as being 'in the way' or setting limits but very much saw me as the one who made the rules, so he thought I had to go. HE didn't realize that I may be the one who verbalized the rules, but my hubby and I agreed on them beforehand or I didn't say them at all. My son's violence toward my daughter was because she was the second child and he was the first so she was the 'interloper' in his mind. I found my son strangling her one night and my son ended up in a psychiatric hospital for several months, but he was a minor. Until we got a bed for him, my daughter slept with me and my husband slept on the floor outside the door so my son had to wake him up to get to my daughter and I.
It is time to give your hubby a wake-up call. And to see a judge about a TRO. A temporary restraining order. If you can, next time your son blows up, get audio or video of his threats to kill you and of him calling you names. Also get video of his drugs in your home - this is a threat because it draws undesirables to your home and could threaten your life. So you can get an order of protection or a restraining order on these, or based on what the officers reported on the blowup due to what happened before Christmas. This will mean he must find somewhere else to live. IF your husband wants to go with him, let him go with him. I am sorry to say that, but honestly, what marriage do you have if your husband chooses a drug dealing thug over you?
In a marriage you choose your spouse over your kids unless the spouse is harming the kids. In this case the 'kid' is an adult who refuses to grow up and is abusing everyone in the home including himself. If he is ANY kind of drug dealer, he is making good money and can support himself, he just doesn't want to. Let him go and support himself in whatever manner he wants to, on the money he is making.
Don't bring the cops down on yourselves. If the cops think your paid off home has been used in drug dealing, they CAN and WILL seize your home. This means you LOSE all equity in it and have to start all over. Will your son pay it back? Say he is sorry and mean it? No one both counts. I know families that this has happened to. Where a drug dealing adult child ended up costing retired or soon to be retired parents their home and most of their vehicles because they allowed that adult child to use their vehicles. The adult child stored or transported his drugs or sold drugs from these properties or vehicles so they were seized and the parents ended up stone cold BROKE with almost nothing to fund their retirement when they had very little time to build up money for retirement. One lady ended up working for Walmart for fifteen years after her adult child's arrest because she had just put in for retirement and her employer wouldn't let her undo it because her adult child's arrest somehow 'tainted' her reputation.
I don't think that you want to go through ANY of that. Get your son out. Go to the courts now. Tell them that you feel UNSAFE with your son there, that he has threatened you with a knife in your bedroom, that your husband supports this unsafe behavior. IF the courts won't support you, get a smaller home of your own and let your hubby deal with your son all on his own.
A therapist and Alanon or Narcanon are excellent ideas. I truly, deeply, 1000% support them. But PLEASE, PLEASE, this is an incredibly dangerous situation. Your safety MUST come first. IF your husband won't put you first, YOU must do it. This adult infant cannot be allowed to harm you any more than he already has. In time, if you want to establish a relationship in a public area where witnesses can see his behavior, where you can set up a video camera or you know a business has a video camera so you have some assurances that you will be safe, you can do that. But there comes a time where sanity must prevail, even when it is your child who is the threat to you. And it sounds like your son truly IS a threat.