Most of the lore is written about the Horus Heresy. From the point of view of humanity, it’s the worse thing to happen to the imperium, as it has not and will never recover from it.
There’s another human civilization called the interex that was basically the anti-imperium in that they were pluralist, secular, and sought peace within the galaxy. The imperium destroyed them, so it pretty much crushed any alternative system or hope in the universe, so that was pretty bad.
As far as actual body counts and gorey details? I don’t know, the eldaar, dark eldaar, old ones, and necrons are all pretty fucked up.
It’s normal.
Especially if you’re writing in a popular style, tons of musicians just use the same chord progressions over and over .
Once you start layering things on the melody, it’s pretty unlikely that it will resemble anything too closely.
As long long as you’re just using your own creativity, I think it’s very unlikely you’ll just clone a song.
If you’re really worried about it, you can just change a few notes in the melody on the page in a way that you wouldn’t think to sing “naturally”