This is a move very easy to explain: they thought they would be useful for them in the future. They feel no difference who kills whom, for as long they do so for their benefit.
Western overtures with Saddam Husain would be a great example. Second to US granting asylim to all batshit insane military mutineers in hopes of using them any time the new freedom fighters don't dance their tune: Haftar Khalifa, Husain Altaf
The West has adopted a very orientalistic concept of war as of late, as of it being purely symbolic bloodshed and ritualised sabre rattling just to compel "tribal elders" of another side to talks on your terms.
Bitter lessons from European genocidal, uncompromising warfare to the last drop of blood has been largely forgotten.
The next time the West will encounter an opponent who treats the war seriously will be its own 6 day war.