So I’m achievement hunting Skyrim, on my best save, I have 10/15 daedric artifacts needed for “Oblivion Walker”. So I don’t want to make a new build, but for the quest “Pieces of the Past”. I need Silus Vesuius alive, I kind of killed him right when I got the quest, thinking I could just take Mehrunes Razor for myself. Well I just realized (42 levels later) that you can’t do that. Is there anyway to get the Razor without starting from scratch?
You have to finish Pieces of the Past in favor of Mehrunes Dagon to get Mehrunes' Razor through regular means. This means killing Silus at a certain part of the quest. If you're not playing on the PC, you'll have to load an earlier save before Silus was killed.
If playing on the PC, you can use the console command,
player.additem 240d2 1 to add Mehrunes' Razor to your inventory.
You can also try reviving Silus. I am unsure if this will allow the quest to start normally, though. Use the console commands
prid 240CE and then
resurrect 1. Silus should go back to life after entering those commands correctly. If this does not work in starting the quest you might have to try console commands to advance / reset the quest to its correct stage. See the linked UESP wiki articles on console commands and quest stages of the Pieces of the Past quest for more info.