He basically has to turn himself in. Even if no one there can force him to, there are FAR too many witnesses for the authorities to ignore. Either he'd have to kill the 50+ people there, or be on the run for the rest of his life. Considering his casting speed, and the more knowledgeable mage right there who could interfere with his spells, the townspeople could easily just hang up on him before he could kill them all (or at least run away to report it elsewhere).
The protection of noble status only goes so far, and attempted mass murder isn't covered (at least when it's that public). Authorities would definitely cover up lesser crimes, but acts like that could spark a revolt, which isn't worth the risk.
Even if the government's authorities wouldn't go after a noble, the guild would definitely have the right (and manpower) to capture and punish one of their members, and they would be encouraged to do so in order to increase public trust and get more job requests.
Sure, MC should have stayed to make sure he did, but he'd be a moron to try anything else in that situation.