I don't think the play really has a 'villain'. For a while we may be tempted to regard Gar's father in a poor light but that's because we are seeing him principally through Gar's eyes and Gar feels bitterly resentful towards his father for different reasons, some articulated, others not. However, as the play progresses we take in other perspectives on S.B., Madge's for example, and we can also judge him for ourselves. By the end of the play he seems almost as tragic a figure as Gar himself, trapped in his inability to express emotion or show love and affection. We see that Gar's emotional shortcomings are not due to deliberate cold treatment from S.B. but rather to a perhaps inherited failure on his father's part to connect in meaningful ways with those around him, including his own flesh and blood.
As for other characters in the play, especially the men / boys, they are all flawed figures in some respect or other, which seems to be part of what Friel is getting at in the play, the inadequacies of the Irish male, especially in the matter of expressing feelings, but to go as far as considering any of them a 'villain', well, I wouldn't anyway.