Maybe it's not actually an attack vessel.... I remember an analogy from a book; say a microscopic - intelligent - space bacteria discovers a satellite. It explores inside, and kills itself by tripping an electic current. And it thinks 'this is a test, I'm dead because I failed it'; but it's not, it's just a satellite.
Maybe the Lucifer - and the Shivans - is like that. The Shivans aren't aware they're killing civillisations and destroying planets, because they perceive things in an entirely different way. Maybe they're just mining energy (for example); attracted to conflict and explosions without regarding them in the same way humanity, etc, does.
And soforth.