Most people want to be good guys and gals, thankfully, a sign of the human nature developed in the hundreds of thousands of years we lived as hunters and gatherers, later partly included in (and partly excluded from) ideologies/religions when circumstances of life had greatly changed and left the humans with a lot of social, economical and mental problems.
Of course, one has to factor in the nature of video games as escapism.
Playing an evil character has a draw to it for someone who is good in life. As they can experience something new and different to what they're used to.
For example, I have no desire to be a criminal... But playing games like GTA, Thief, The Getaway, Mafia, Hitman etc. Is fun. It allows me to experience something different.
Would really anyone worship Shar for example of follow the dumb BG murder ideology stuff?
Oh absolutely.
Not only do we have people in real life who follow "Questionable" ideologies. But in D&D settings, there are actually tangible benefits for siding with evil gods. Like, literally you can be a Cleric of Shar and get cool powers.
This is often true in other settings. In fantasy, there are often actual gods (Or god-like entities) that bestow gifts upon their followers. Thus adding some actual logic behind faith in them, as opposed to real life where faith is just based on feeling as though it is real.
On a side note;
Something that often bothers me about games and their morality systems is that it's quite often just a binary thing. You're either goody-two-shoes paragon of the people, or you're Jerky McJerkFace the murderhobo. There's no inbetween, no neutral or morally grey representation.
It's especially apparent when looking at the Star Wars universe... I hate both the Jedi and the Sith. The Jedi are way to uptight and annoying and the Sith are far too "Chaotic Stupid". There exists the middleground, the Grey Jedi which are more morally ambiguous but they barely ever, if at all, get any screentime.
All this get compounded on by horrendously awful implementations of "Morality Scales" which basically force you to make all "Good guy" or "Bad guy" options in order to build up enough morality points to select options later on... For example, when I did a full playthrough of Mass Effect, my Paragon character took every Paragon option except 2, once in ME2 and once in ME3 (They weren't even "Bad" options. They were either logical or simply human) and as a result I couldn't get the Paragon ending in ME3...