Never rly understood that part tbh. Even if my take here might be quiet controversial, since most seem to have some kind of deranged murder fantasies where everyone who even slightly wronged you just has to instantly die a horrible death... i personally liked Serena and would´ve hoped to see her again. Sure, Eliza got hurt because of her decisions, but she would´ve done the same for somebody she loved. And in the end it´s because of Serena that Eliza can be with the MC for eternity. So yeah, i´m gonna say it, the ungrateful bitch should thank her for that instead of trying to kill somebody who just wanted to protect his family.
Edit: And judging by the text and the H-Scene, MC was willing to forgive her, he only wanted her to make amends with Eliza, which Eliza never accepted, like a petulant child.