I saved Silus now I want the razor :P Because my character turned bad. I have PC so console commands would be cool :p
As far as I know there's no way to go back on your decision to spare Silus and get the Razor instead. If you're playing on PC, however, you can just give yourself the Razor with the console command
player.additem 000240d2 1. Killing Silus before or after doing so is optional.