Poe was Ben's childhood friend and/or sweetheart. When Poe's taken by the First Order, the Storm Troopers rape him to try to break him down. When Kylo enters to interrogate Poe himself, he notices Poe's clothes are in disarray and realizes what happened. Flying into a fury and massacres the rapists because whatever is left of Ben can't tolerate Poe being hurt in that way.