So I'm playing evil character and now since the gatekeeper Bill attacked me and I have killed him I can't enter Ducal Palace. Is there other way?

You can respawn him by typing this command into the console:

CLUAConsole:CreateCreature("bill")

You can copy that line and paste it into the console; you don't have to manually type it.

Then just talk to him again. He will be created at the center of the screen when you enter that command.