Toffee is a good guy.
Mewni is evil.
That Independence Day stuff was quite obviously propaganda created by mewni royalty.
Monsters have been discriminated against for too long, and due to this Toffee used aggressive methods against moon and the magic council. Peaceful methods are only an option when the other side is willing to negotiate or are open to other ideas. Monsters face racism that's deeply integrated into the Mewni Culture, as 'Monster Love' is considered taboo/outlawed in Mewni. Even Eclipsa, a queen, had to leave.
Moon allowed Star to Date the son of 'the big guy', and it's pretty obvious that is meant to be this universes Satan (who's last name seems to be Lucitor). This shows Mewni has good relations with what is the equivalent of hell (called the Underworld, for goodness sake).
Good guys aren't associated with hell, or are friends with Satan.
Also, these bugs just look evil/unholy abominations.
TL;DR: Monsters did nothing wrong.