Nefcy has compared Toffee to Magneto, as a villain who has a point but goes about things the wrong way.
Other crew members have alluded to or directly stated that Moon and Toffee have a sort of tragic history.
What sort of relationship do you think they had before they became sworn enemies? Or did they know each other at all, beyond the battlefield?
I don't buy into the romantic angle, it's too weird with too many problems even beyond the weirdness.
I'm predicting that Toffee was representing his clan (or even monsterkind in general) in some sort of diplomatic mission to the Butterfly Kingdom, trying to negotiate a lasting peace with Mewmans and/or earn monsters better treatment or a recognized sanctuary outside of the Kingdom. Princess Moon would attend these meetings, and they went on long enough that she came to respect and sympathize with him and monsters in general. Maybe Toffee even had his own children around Moon's age.
Of course, this would end with the Queen and King double-crossing Toffee somehow. Things get ugly quick, most of the monsters are slaughtered, but Toffee obviously survives and strikes back with a vengeance, leading to the death of the Queen.
Young Queen Moon is bound by duty and honor to defend her kingdom and family, in a conflict they started against people who just wanted a chance at a better life, from a once good man driven mad with grief for his own loss and the rage of betrayal.