It's really not his or her job to fix abusers.
most societies believe that each individual must contribute to some degree. It's being lost, but for most societies historically the mentally ill were the burden of the immediate family.
Now we see the effects of the mental illness affecting society-at-large, it stands to reason that the entirety of the affected population is now somewhat responsible for making their own lives better by trying to assess and remediate the problems within their society.
If I walked past litter on the ground in my home-town, I wouldn't say "Hey, not my job." and walk past it, I would pick it up for the betterment of my town.
This isn't a foreign concept for most people.
This is this and that is that. Litter won't stab you, whereas I've had a friend get stabbed by a mentally ill person who asked for their restaurant leftovers, and were given them, only to then do just that. (Which is a bit different than what the OP is talking about, but you mentioned mental illness in society in general, so it seems fair to bring it up.)
If for 3 years I am constantly picking up litter as fast as other residents throw it on the ground, at what point to I just move to as different town where people litter less?
That sounds like tax money to be used for mental health services.
Even societies "without" life imprisonment have legal ways of putting people away for life if they're deemed too incompatible with society. Take Norway; longest sentence is 21 years, but for especially vile crimes, that sentence can be extended at the end of the 21 years if the convict is found to not have been rehabilitated.
this strikes me as similar to the concept of being for life imprisonment over the spending of resources towards rehabilitation.
To throw people to the wayside because YOU have determined them to be 'UN-fixable' IS wrong. It's useful for YOU to compartmentalize them out of your life because they represent a personal burden -- but this doesn't help them improve themselves or their conditions; it may not be your responsibility to help them improve -- but many of these people are hopeless without outsider intervention, they need help, not partitioning. If you can't help them then the next best thing is to make an effort to find them help, not to shut the door in their face.
>It’s more wrong to let them perpetually abuse people
what do you think happens when you make yourself lost contact with these people? Well, having been 'compartmentalized' earlier in life and never receiving any help or social training, they continue to perpetually abuse other people.
Ironically the strategy of isolating them from oneself likely increases or prolongs their time 'out of step' with society -- all it does is protects YOU from the damage the person might do.
All I can say is that i'm glad the entire world doesn't take your advice of 'compartmentalizing harmful people', otherwise this place would be a pretty brutal world to live in.