soup makes some valid points but I agree with RiSC. Break it down to it's simplest point.
Forcing a living creature to attack another is wrong. Free will is for humans. Animals by nature will flee or fight when scared or provoked. Forcing them into those situations for sport is wrong. Just like we don't chose our parents, dogs don't chose their owners. If they could they wouldn't chose to be tortured by Michael Vick.
We slaughter animals for meat to eat. The means of slaughtering pigs, cows, lambs and others is not very humane but the result is to feed humans. Dog fighting is only for gambling. If they really had any balls would fight Kimbo for money.