The question remains, why are Shar or other evil gods worshipped? They offer nothing special to their followers what not other better gods can give, and what the baddies offer generally is destruction, period. For example why would you sacrifice your child to a god (like in the case of Baal-Hammon as worshipped in Carthage) if all he/she wants in the end is destruction?
In the human context really bad gods like Shar simply don't exist. The gods of DnD (and admittedly quite often other fantasy universes) often appear to me as if created by a group of 20 year olds during lunch break. Shiva for example is a totally different kind of god than Shar. But here is not the place to discuss such things. So I let it be (you won a discussion, great, isn't it?) and live with Shar et al.