Here is a thread that popped to mind that is on point.
http://shatteredkingdoms.org/forums/vie ... 24&t=21834Short answer, no, I don't think it's appropriate for a light aura to act evilly off the bat.
Longer answer: The character you're describing sounds like a grey aura, who have a great deal more leeway in HOW they act, but also require a lot more work to demonstrate WHY. "Because I'm a good guy" or "Because I'm a bad guy" frees the alignment extremes from (at face value) justifying why they do or don't do things but locks them into behaving like good guys or evil guys. Make sense?
If part of your principled paladin's backstory is being raised in the Moot as a barbarian, OK, work it in carefully - but upon coming of age and heading out into the world as an amateur adventurer, the conversion should either happen before then as part of his backstory, or be the 1 thing you focus on above all else until it's done at Amateur. Picking principled and then going into the game and saying "well, I'll just act miscreant until someone converts me" wouldn't be appropriate, I think.
I think this sums it up pretty well. A lightie who used to be a horrible person should have gotten all of that out of their system by the time they enter the world.