Verbally abusive, alcoholic 19 year old son

I'm a single parent of 2 teenagers.  My 19 year old son is causing much strife in my household, enough so that my 16 year old daughter no longer wants to be in the house as she is scared and tired of the arguing.  She is constantly staying at friends houses to get away from her older brother.  

My son has not lived an easy life.  I've made some decisions along the way in his life in an attempt to help him deal with some emotional issues.  Those choices were not the best ones and as such, he's been on a path of destruction since the age of 13.  He's been abused by the professionals who were supposed to help him.

We are at a place right now where he is unemployed, not in school, doing drugs (marijuana) and drinking alcohol.  We live in Canada so he is of age to purchase and drink.   He also struggles with bouts of suicidal thoughts.  His behaviour goes from one extreme to the next - he can be a very loving and respectable young man (he was even working at a street outreach clinic helping the homeless) and in an instant be physically and verbally abusive.  

I come home after a 10 hr workday and there are thugs in my house.  They are drinking and smoking marijuana.  When I tell them to leave my son becomes verbally abusive.  He outright refuses to leave and talks to me with no respect.  If I threaten to call the police he grabs a knife and holds it to his wrists.  And there is no bluffing him - he's slit his wrists in the past with similar incidences.  It's very painful thing to watch your son hurt himself.  There are holes in my walls.  My front door is broken.  I constantly have migraines from having to deal with his destructive behaviour.  I have neighbours complaining constantly.  

And I don't know what to do anymore.  

He refuses to get help, as I can understand, our attempts at receiving help in the past have failed and betrayed him.  But I can't cope with the abuse anymore.  

[RockRose]

It seems out of the question to keep putting up with this,  since you have a 16 year old daughter who needs you and is basically homeless due to his behavior.  Otherwise it would be a reasonable alternative to wait this out a while.  

Is his father living?