Just cause they want to make the universe the best it possible can be by eliminating anything weak, it kinda makes them seem less evil in a way cause to them it's the right thing to do. Like they don't think what they are doing is bad. But I suppose no ones that is classed as "evil" thinks what they are doing is bad.

I haven't read all of the Books of Sorrow yet, but the impression I got was that the whole "rid the universe of weakness" is an excuse. The Worms (therefor, Darkness) are using the Hive to wage their war on the Light. Rope a species in with promises of immortality and power, make the unspoken cost of the deal eternal servitude and corruption, and pretty it all up with a silly little bit of philosophy. In one of the cards (XLIII) Oryx meets something like a younger version of himself via Vex, and it is absolutely horrified.