"Something got in the way of my vengeance"...
Seems seems pretty obvious to me that his future self that is filled with regrets went back in time and fought him to stop the plan that would inevitably push her to end it all.
Like, you get his thoughts in the chapter titles, and his actions are contradictory. It's more like he's actually feeling empathy for her but still resent her, so he's trying to convince himself that he's not helping her out of kindness. He's being conflicted.
It seems like MC has been planning to take revenge on her but is having a change of mind. That is obvious to me because a revenge ending wouldn't feel right, but I might be wrong.