In a situation like that, you would naturally have at least some monsters who wouldn't trust the treaty to be worth the paper it's written on. Any rights that the Mewmans grant to monsters could just as easily be taken away in the future.
But at the same time, they'd see the fact that the Mewmans were resorting to a treaty at all as a sign of weakness. Making it the perfect opportunity to strike.
Afterward, Toffee realized his mistake. He already knew the obvious, that the entire foundation for Mewman dominance is that they have magic and monsters don't. But you can't remove that advantage simply by assassinating the queen. The magic would still exist, and there would be others in the royal family to weild it. And the wand would still exist, potentially empowering even somebody outside the royal family. But if magic itself were eliminated, Mewmans would be permanently removed from power.
And this would be especially advantageous to Toffee personally, since his particular species of monsters would be nearly invincible without the threat of magic to nullify their regeneration. It's likely that without the common enemy of Mewmans standing over them, the monsters would fight among themselves for dominance. And the Septarians would have a massive advantage in any such fight.