I think if the writers want Tommy to turn evil, they would need Tommy to suffer some terrible loss by Oliver's hands. The writers need to do something like what Joss Whedon did in Dr Horrible, kill off Horrible's love interest. And I don't think the writers would ever kill Laurel. I suppose the writers could kill off his father as the reason for his turning, but don't think Tommy loves his father enough to go all evil.
Perhaps if it turned out his dad killed his mom? Wouldn't give him the reason to hate Oliver, but the shock and trauma could be enough to push him over the edge. Again, he does not have to be evil to be the hero's adversary.