I don't think Neil deserved what Ria did to him at all. I don't think Ria deserves what Neil does either. If anyone is to be blamed here is ND for sure. ND absolutely must have done something to create the MU between them leading to Neil's mental illness and Ria's thirst for revenge. Neither of them deserves what is happening to them. Just like SidNi, Ria and Neil do love each other immensely but their love has been turned into this hatred for each other due to some MU.
This is what I think ND might have done:
Told some lies to Neil about Ria. Neil at first not believing her. ND creates situations to make Neil believe her. Neil's bipolar side comes out and the fear of losing Ria comes out. ND tells Ria about Neil's aggressive side (but not that he is bipolar). Ria sees this side and feels hurt and betrayed. Ria leaves Neil thinking he is not who she thought he was. Neil thinks Ria left him for someone else and betrayed him.
I am 100% guaranteed they are both victims of ND's work to keep them apart for whatever reason. I don't think ND even wants her son to be happy. She kept the truth about SidNi's relationship from him which is wrong on every level. If anyone is to be blamed it is ND.