I think that we've just happened to see "nice" gifters so far and that gifting your powers doesn't affect you at all, it doesn't remove the evil but it doesn't increase it either. There very well might be something special about them but I don't think it's that gifting reverses the evil, partly because if it did, wouldn't Prof be a very nice guy? He has gifted three different powers at least four times, so that's twelve giftings and he rarely uses the powers himself. Unless using them makes you evil at a quicker rare than gifting reverses that.