Oh, I don't think the defense of a slice of life world is a bad idea for a plotline. I'm more saying, in a crossover RP, having a lighthearted well-liked setting of any kind mercilessly destroyed with no real chance for intervention is kind of dumb.

As for ways to show how evil the villains are, it's equally possible without, say, having them somehow know that people like certain characters and killing them literally just for evil with no actual motivation. Like, say, having them deal with their enemies in merciless and cruel ways, opportunistic methods without regard for innocents... Deceiving people into working for them...

In the end, Shadow Eternity is [i]not[/i] in the business of large-scale conquering so I'm not sure it makes sense for this particularly. There'd have to be a specific object the Leader wants for her ultimate plan on a world for her to send people there, though her intentional vagueness probably makes it easy for people to assume that she wants some conquering to go down.