Yes, the betrayal was about Marcus, but Maria didn't even really love him (as she literally tells him at the end) she was just using him because she was messed up emotionally.
Maria was mad at Saya because it was a betrayal. You can hear the real reasons in their dialogue during/after the fight. I understand your opinion, and I think that their fight (while some of my favorite fight choreography of the show) was excessive compared to its comic counterpart, but I don't agree that their fight was really about Marcus at all.
