This is from the "intro" of the novel talking in general about sayayins . It seems that is like a sort of poetic/archaic style as the grammar doesn't make sense to me.
I think it means
Their liking is the battle, to invade other stars [they] were the hooligans of the universe.
It's that is correct, what is the grammatical explanation for the connection between this [ほかの星を侵略する] and this [宇宙のならず者だった] as there is no と, も, commas, pre-masu or て-form linkage.