Nah I think it was part of the whole "Osiris nd my wife betrayed us so let's fuck em up nd get revenge on him by ruling together". Also, it was that he thought Nephthys would only love him if he was king nd had some worth, that's why he wanted to be king nd to be king he should have the queen too. He's angry at Isis cuz she isn't helping him in that revenge, instead she's taking her own revenge on Seth. Soooo yeah, Seth was only hungry for Nephthys kitty ig (not anymore tho cuz of yknow...Horus's glowstick)

And is Seth mad at Isis because she never gave he any her kitty
because in chapter one when seth killed osiris he went to her and demanded they share a bed (⊙…⊙ ) but she didn't want him