If you're a Shaper, you can easily kill him if you dumped a good number of stat points into creating Fire creatures. But yeah, he doesn't do anything special. You just get a huge boost to your exp.
I'm guessing that you can't return to fight him later because the tutorial is a special event. But if you want a story explanation... Everyone ran away! If you were a servile, would you stay behind and get slaughtered by rogue creations? Probably not! The thrasher just accepted his new found freedom and ran for the hills.