I doubt it has anything to do with killing people. I think the system works by accumulating points on a spectrum from hero to villain, these points being generated 100% by dialogue choices, and to trigger the event that gives you the particular tag, you must finish a quest capable of granting either in a specifically villainous or heroic way.
So to get the tag from Migo, you need to first accumulate points and then finish the quest as a hero. It would explain why I didn't get the tag at Migo; we likely didn't have enough points yet, and the next opportunity was at the Reaper's Coast bridgekeeper.
Either that, or the entire thing is bugged to shits.
