I recently decided to have a bit of fun by slaughtering everyone at Hoover Dam. I was playing NCR, so I was going to reload to my last autosave. Unfortunately, it autosaved to a point AFTER I started killing everyone. Is the second-to-last autosave deleted?

The game only saves the last autosave created. So if you walked into two rooms that require loading screens, the second one is where the game keeps the save.

Better beg to ceaser

Why not just make a hard save before doing that?

^^^^^^

This is pretty much a cardinal rule of playing RPGs, learning when to save when you decide to do something wildly divergent that you might not want to continue playing but just have that curiousity as to just what would happen if you did that wildly divergent thing.

Case in point, my current character is going to have as many of the side quests completed as can possibly be done before even initiating the second chapter of the main quest. Gonna be a while before I say "hello" to Benny with a solid "goodbye."