Everybody has the ability to do 'magic' but not everybody has the knowledge of how to access the power. If some-one who has the ability tells some-one else how to access that power, and that some-one was say, for arguements sake, a slayer !.
I haven't come across the black-eyes thing before, it may be just a TV thing, letting the audience know that the person is evil now. Now if it was about light-blue eyes, as blue is the colour of power.
As to who would win, the evil side or the good side, for the good to win it would need to adopt the evil sides will to kill, so the time factor would come into it. The evil would just do it where-as the good may hesitate.
Using the boxer theme, if both boxers were of equal strength and they were allowed one punch would the winner be the one who pulled his punch so as not to permanently harm the other or the one who wasn't bothered if he killed the opponent.