I don't know way Molag Bal is instantly seen as pure evil. I find him interesting, as well as terrifying to be honest, because I don't completely buy into the idea he is simply the Prince of Self-destruction and Corruption. I've had this idea in my head for a while now that this interpretation of Bal was created by Aedric worshipping cultures, mostly the Nine Divine church, to make him unlikeable and simply stuck. But really I see him as the Prince of Self-liberation and Freedom which Aedric religions couldn't tolerant as, like the major real world religions, believe that doing the "good" thing requires sacrifice which I believe Bal would see as pointless and instead encourage the "virtue" of selfishness.
I just have this feeling that the authority in Tamriel is just too extreme in warning against interacting with Molag Bal that I think there must be an ulterior motive for their warning. For instance, in Morrowind he is said to be corrupting the bloodlines and while that sounds insidious it could easily be seen as Molag Bal supporting inter-racial love (awh). Even the whole corruption thing Bal is connected with I think are leaders fear that Molag Bal's influence corrupting and ultimately destroying their regimes. I imagine Molag Bal seeing regulation of anything as the weak holding back the strong, a very cold way of seeing the world but at least in a sense fair. So depending on your view Bal is either the embodiment of merciless anarchy or genuine freedom for all. Quite a contrast but at least it makes him a bit more deep and interesting than simply a god that wants to "r-word" any hole he can find.
SUMMARY: Molag Bal is an objectivist