In as parochial and emotional a construct as football, seems like as good a reason as any.
Well, no, it isn't. It's a stupid, childish reason.
Does he know the guy personally? Did he wrong him in any way? Probably not. I dislike some football players, but only because of their actions. You don't have to like the guy or anything, but it's extremely petty disliking someone you don't even know personally based on the sole fact that they play at an opposition club. Stupid, really.