I can't give any suggestions regarding where to look for a fighting style, but hows about this:
You said the spear seems to "encourage" your character to desire power and tend toward the evil side of things. This suggests to me that maybe it's intelligent, or cursed. If so, I don't think it would be out of line to think that maybe the spear is beginning to impose its will upon your character, thus changing his fighting style while he uses the weapon. Think of it this way: if he was never a spear-using kind of guy, he suddenly gets hold of this spear and seems extremely proficient with it. Or, if he was a spear-using fighter before he found it, maybe it alters his style a little to suit its own needs?
Just a thought...
~Box