Honoring an abusive parent

 Recently met someone who had a dilemma. We all know honoring parents is a critical mitzva. But is it required to be observed even if it causes pain and anguish?

In this case the parent was not only abusive physically and emotionally but was not only non observant but strongly and ideologically opposed to Judaism, The issue was whether to say kadish for his Yahrzeit. Halachachically there is clearly no requirement to say kadish but he wanted to say kadish even though it caused him great anguish. Is he doing a mitzva by saying kadish?

One posek responded

"He can say kaddish and is also doing a mitzvah"