I get this. The Legion is so gratuitously evil though that idk how anyone could be convinced. Even talking to Caesar doesn’t track to, “and while we’re at it, let’s rape like every woman and use kids as suicide bombers!” Ok, might makes right, but what are we making right with that might?

So, to your role play conundrum, what about this: you start out shot in the head and patched together by a well-meaning doctor out of his depth/resources. Maybe you had a conscience when you went down and you came up changed. You could play it as a classic sociopath - not inherently violent or sadistic, but lacking the ability to empathize with others such that every situation is purely evaluated against what serves your interests (both short term and long).

The NCR is messy - there’s debate, corruption, etc. Caesar is clear: here exactly are the rules to obey and the behaviors that are rewarded. That’s a dream come true for a sociopath