What's the difference in situation between Imperium Nihilus and Imperium Sanctus?

I don't know the details of the overall state of the galaxy, but from what I've seen on social media and what not, apparently Nihilus is the "crappy" or at least "crappier" side of the galaxy. I'm interested to know since I'm a huge Blood Angels fan and Dante (favorite character) got put in charge over it. The Dante trilogy says he's trying to completely secure the area in the Red Scar to set up a "main base" for Nihilus, but thats all I really know. So what makes that side the bad half to be on?

Side question: Guilliman appointed Dante as Regent of Nihilus. But since the Lion has awaken and being a primarch, wouldn't he assume the position from Dante?