For these things in our world, you have to look at them as if you were a person who lived during those times. Stuff like that was not evil then, it was just how things were. Much of history is written by people who have particular biases and opinions of their own, and a lot of the gods were converted into demons or something evil later on to try and discourage their worship. Also, the gods of our world often had a dual nature, in games they are often simplified a bit more. A lot of the FR deities are based on a particular version of ones in our world but they are still quite a bit less complicated, probably because they have to fit into alignments. As for why evil game deities are worshipped or at least honoured in some way, there are many reasons people could have. Power being one of course, but someone who has lost everything could easily turn to a god who promises revenge or one who claims to ease the pain of loss. Fear too can be a strong motivator (sailors making offerings to Umberlee for instance.)
In history, people sacrificed children in fires and murdered virgins on altars. For their deities, they raped and abused others and killed them. Common people worshipped them out of fear or coercion, and those who instituted the religions did so to create said fear and coercion. It was to control people as well as because they simply enjoyed being psychos. They got their kicks out of hurting others.
My point is that the same is true for FR. It's no different. People worship evil deities either because they are seeking power, they're duped into believing it's good, or they're coerced through fear or violence.
Wasn't the question something like "why would anyone worship an evil god?". So my answer was, "History. It happened. Lots of people worshipped evil gods for many reasons throughout history. Why would FR be any different?". I also believe that it was presented as if WotC sat around making up these gods as if something similar never existed in the real world. So I was pointing out that gods like Shar did, in fact, exist in real human history.
