I reckon the French Foreign Legion offers a great prosocial container for psychopaths. Considering the only psychological entry requirements are an IQ test (irrelevant for psychopaths) and a personality test (very relevant, but easily gamed), I reckon they could get in asssuming they meet the psychopathy indifferent characteristics.

Considering that psychopaths may indeed be prosocial or ethically motivated. They might not be very happy with regular life, so socially legitimacy in the Legion would be a superb opportunity for them to advance prosocial causes, relative to say criminal violence.

No one much joins the Lions of Rojava cause they have shit recruiting, propoganda and social media management.

I reckon people fight for the sake of fighting, not for ideology

Ahmed, who fled his town near Raqqa in June, said some of the Arab fighters would try to mix with the local population, but the Europeans and other non-Arabs never did. He said that although the Islamic State militants claimed they were there to create a better life for Muslims, they seemed mainly focused on battles with other rebel groups and government forces.

"They were always very aggressive, and they seemed angry," he said. "They are there to fight, not to govern."

they're frequently described as ''oversubscribed'' and

The FFL currently has 7,700 troops. By way of comparison the California National Guard has 18,000 troops. The FFL only takes 1 of 40 qualified applicants according to their own statistics. And the large majority of those they take have previous military experience in another country.

Perhaps that explaisn the popularity of foreign fighting with ISIS

I suppose the main reason it would be unethical to recommend this line of work even to psychopaths is the rate of injuries like back pain and such, even for those who don't go to combat (training).