MYou see the world not as "Good" vs "Evil" but rather as "Village" vs "Magic". I simplify and there can be a lot of role variations, but I had zero problems attacking goodie mages across three or four good villagers. Probably hitting/camping Fort is not ok, but attacking a ranking goodie mage is fine. (Don't follow Korsgaard).
Some examples:
You're trying to get the world rid of the "greatest evil" of Magic, and our mage friend over there has kinda crossed over on the dark side and until he's stopped, your job is not done.
Or you're trying to kick the crutches of Magic from under those who rely on them in order to help them stand up for themselves.
Or you're not really trying to kill him, but only to stop him using Magic (use no lagging moves).
Or you're the mightiest warrior of all of Thera and damn those who think they can stand in your way. You'll challenge any and all, especially those who think they are better than you just because they're using magic. It's not about killing them, it's about proving you're better. If they die, oops.
Or you "tell goodie Stay clear of me, you foul creature" and if he doesn't clear your presence, then it was his wish to fight you.
Or just skirt around goodie mages, which is not hard to do as there aren't a lot.
Be willing to RP significantly more than usual silent killer types though. And role is pretty much required IMHO. Oh, and obviously things like remorse and not killing goodie NPC's go a long way.