I refuse to place Scar in the "enemies" side just yet. It's true that so far all the encounters between Scar and Roy haven't been really peaceful -- in fact, both of them ended up with a fight. Still, I'm biased towards the guy, and believe that he's not a bad guy at heart and might be recoverable.

The truth is we can only imagine the hardships Scar may have had to deal with during all his life. It's perfectly logical to hold such a big grudge against the country's military, and especially the national alchemists, since they were the one who ultimately wiped out his home, Ishval. Scar and Roy's relationship, therefore, is totally undermined by the tragedies of the past.

     

Scar: Alchemists are thosethat twist the natural state of creation to a degenerate form... that is, in short, sacrilege to our creator, God. As the messengers of God, we are those who carry out God's judgement.
Roy: That's something I don't understand. There are many alchemists, but why are you targeting only those that have national licenses?
Scar: No matter what, if you intend to interfere, I will eliminate you too.
Roy: Interesting!
Hawkeye: Colonel Mustang!
Roy: Don't lay a finger on him.
Scar: Mustang... as in the national alchemist?

Of course, there's probably not a day Roy doesn't hate himself for what he did at Ishval. And Scar is slowly realizing that his hatred brings only more hatred in return. Despite the seemingly irreconcilable situation between the two, I still think there's a possibility that they'll reach an understanding, and perhaps start working together against same enemies.