User blog comment:Miskos3/Fairy Tail Episode 248: Tartaros arc: Attack of the Celestials, Review/@comment-186.45.168.252-20150830022311/@comment-7418318-20150830055329

He didn't do it out of mercy or empathy. He just thinks that torturing is a waste of time and would prefer to finish his missions quickly and efficiently.

That's probably also why he backed down before Jackal, since a fight between them wouldn't have gained them anything.