There's only an argument for altruism and cooperation if an agent is certain the other agents are running a decision theory that is as good as its own and their utility functions are similar enough to cooperate.

It also works in society, with a government enforcing law and puts forth other fines and incentives. Furthermore, humans are messy. Our immediate desires, on a lower level, can be quite different from our long-term, reflected, higher goals. It's a matter of impulsiveness which of these wins how frequently.

I don't believe psychopaths have any greater-utility terminal values than getting what they want at the expense of others.

Why "at the expense of others"? That's a property of the world, not of them (which does not excuse what they do).

I don't want to punish psychopaths either. I want to make them less psychopathic.

I apopogize for misreading you. "We didn't eradicate malaria or polio with kinder, gentler methods. Don't leave anything for evolution to work with." sounds rather more... straightforward to me than you apparently meant it. I'm glad that we agree that psychopaths should be helped, whereever possible, to become productive parts of society.

Therapy appears to teach psychopaths better skills at manipulating people.

Then the current therapeutic methods are not well-suited for this task. If we want to go down the road of developing more effective means of psychopath crime prevention, therapy etc., I suggest we Hold Off On Proposing Solutions.