Asking Roommate to Move Out: Harassment Ensues

My question involves landlord-tenant law in the State of: Washington

I'm the only one on the lease, the landlord is a very relaxed, honest, nice guy and specifically wants me to handle all of the roommate situations.

Long story short, I made a bad call on a roommate and after 2 months, I asked him to move out. I was very plain and noncombative (I hate confrontation) and simply said that it wasn't personal, it was simply just a differing of living styles.

As far as I'm aware, I only need to give him 20 days notice (which I did) but I did tell him that if he needed help with anything, I'd be happy to try to work something out. I don't want to screw the guy over, even if I'm certain he will.

Fast forward a couple days, and I am having someone over to look at the house. Another of my roommates is moving out at the end of the month (he was just a short term while his family finished the school year before moving) so I was having her look at his room, and not the terrible roommate's. Here's where the problems start:

He comes into the kitchen with his phone against his head and begins ranting about various made up scenarios to try painting me as a filthy, ungrateful lunatic. Again, I'm nonconfrontational, so the potential roommate and I go outside. I apologize and begin talking about the yard, and the terrible roommate follows us out, and starts insisting that he has 90 days and is trying to instigate an argument. I tell him that I'm also renting out the other roommate's room at the end of the month, and ask that he please be respectful. He keeps arguing, claiming that "It's his house and he pays rent and he can do and say whatever he wants." He also mentioned "My band buddies are coming over and they want to have a talk with you." In a threatening demeanor. I'm certain this won't happen, he is the biggest peacock of an insecure grown man I've ever met. His band mates are all much more sensible, hopefully they'll talk him down a bit, but still, I'm worried.

If he persists to harass myself and potential roommates, do I have any recourse? I'm certain he won't pay utilities, after I told him that it was time he found somewhere else to live, he actually demanded his rent back (no, he did not pay me a last month's rent, even though I had originally asked for it) but at this point, I just want him gone. Small claims for $140 in utilities just wouldn't be worth it, seeing as he also had thousands of dollars in unpaid tickets that he took years (and was driving on a suspended license) before paying.

Eventually he goes away, but I'm wondering if I have anything I can do to either mitigate his harassment of myself or potential new tenants, or can do if he actually continues these threats, baseless or not?

also adding that scheduling people to look at/visit the house simply when he's not around is nigh-impossible. He has no job (supposedly he has one, but has "worked" a total of two whole days in three months) and is almost always home.