Will Solas Leave the Inquisition?

Solas will always leave you at the end of the game.

Can you kill Solas in Dragon Age Inquisition?

It is possible that you simply can't. So the only way to defeat him is to get him on your side, or perhaps make the choice to die.

What happens if you let Solas kill the mages?

Solas: As you say. Solas: Dareth shiral (Untranslated, means "Safe journey") After Solas either kills the mages or stops himself, he will then return to Skyhold, where a follow-up conversation will begin upon returning. Until then, he will not be available as a Party member.