Basically a Lightwielder suffers from Lightburn whenever he does something Dark. In this pocket universe, the Light is a pure force for good, with no personality or opinions or quirks that we know of. So basically if a Lightwielder kills someone he's gonna die himself from the backlash; if he steals or cheats or lies, he'll suffer various degrees of Lightburn depending on the damage he does, the evilness of his intent, and how far gone he is in the Light. Greed is also Dark, because it deprives others of their fair share of whatever you are hoarding. But you won't take damage from sin against ritual, like eating bacon, or saying *bleep*.
Now the main character is a teenage boy, so I have to figure out whether and how much he would take damage from random sexual lust, because a normal (or even not so normal) teenage boy will be exposed to that almost continually. Obviously temptation as such will have no effect. I'm currently working on the hypothesis that there would be lots of small burns from greed at least. People can be very greedy when it comes to sex. And if he actually made moves to tempt someone into cheating, he would probably get a pretty bad burn, but I'm not sure how concrete these intentions would have to be.
Or I could just play The Sims Hot Date all night again. Wow, those Sim females sure throw their curves around on the dance floor ... OWW! OUCH!