Septarians never forgive nor forget, it's part of their species.
The fact that they live longer only make it worse as they can tell multiple generations about the atrocities of the past.
Solaria used to torture septarians in different ways just to check how long the body took to regenerate, just imagine surviving that... I would feel revengeful for sure.
Globgor tried to convince Seth of prioricing peace, as he didn't want more monster blood to taint the ground, but he refused to trust mewmans after all they'd done.
Eclipsa sure considered them a threat because of this (that's why she created the killing spell and used it on certain individuals) but overall admited that they were fun, interesting and 9/10 boyfriend material as long as you weren't the enemy.
That's why they killed Comet but not the Cat Archduke, because last time peace was on the table Mewni betrayed the monster alliance, so they thought they couldn't trust this "lying" queen.
Septarians hatred towards Mewni is justified but they are far too obsessed with the past.