there is no good reason for your character to be able to slaughter every single thing on Faerun.
Indeed! Especially with there also not being ANY negative consequences for you.
I don't care how powerful you and your party are, there is no way you can just go about killing people, or even stealing from people, and not have everyone in town and all the guards and eventually the whole army of that kingdom come after you and put you down. So this is my gripe with the evil side of things. I absolutely agree players should have the freedom to do things as they wish. But they cannot and should not expect the setting's very nature to be suspended to allow them to have this freedom. If the setting is essentially a "good" favoring setting, which the Forgotten Realms most certainly is (with the exception of certain specific locations such as Thay or Zhentil Keep), then wanton killing of innocent people should generate a massive negative reaction against you.