I would even be suspicious of supervised visits (unless maybe with a whole squad of police...:(); it sounds like he crosses boundaries even with supervision, so your other kids could easily have cause to not feel safe in their home, where they are "supposed to" feel safest -- and not feel like you value them enough to protect them. The safety issues cause changes in our "reptile brains" for many people. Also, not being protected by parents may get generalized by some deep part of our brain into stuff like "I'm not valuable enough even to protect from possible death". Those things can really set kids up to have ptsd now or later; their brain's ability to regulate emotion and stress may be damaged; they might not avoid future abusive relationships; they might have other issues for decades... it's really important to protect them now, their brains are especially vulnerable to this kind of stress as they are still developing. There is a lot of research showing these things now.
Various people here have had ptsd and other issues for many years due to abusive siblings from whom their parents did not protect them... (I'm one of those, there are others that I've seen here.)
I understand that it's very difficult for a parent to feel like they are "giving up on" a kid... The parents often feel guilt too for other reasons; maybe, "what should I have done differently", "there must be something else I can do", "maybe seeing how good people act will help"... then we hear "don't give up on kids" all the time. Which is good! but what does "not giving up on" mean, exactly, in this case?
Letting the older son have opportunities to abuse others really isn't helping him in any way! In a way, it may be letting him feel temporarily better, sort of like an alcoholic getting a drink... (That's how my brother seemed to me, emotionally, when abusing...)
Your 19-year-old is now an adult and responsible for his actions, whatever his disorder's cause (who knows what that was, your other kids aren't abusive...) It is very hard to know what can help certain disorders; being abusive is something that a high percent of people don't seem to decide to change about themselves. Ever. No one else can make them do it, either, and it can be hard to know whether they have truly changed or are just hiding it much better. Very tragic, but possible and important to consider; my brother is now 53 and still at it. I wonder sometimes if facing the fact of what he's done to others is just too painful to heal from and change? For him facing this pain perhaps would be in addition to facing having been abused himself, and the latter is very difficult in itself.
I am very glad you are willing to think about these issues.