I accidently killed Mettaton NEO before killing all the required enemies

I killed Mettaton NEO and then proceeded to go all the way to Sans and talk to him. I stopped there. I'm trying to finish the Genocide route, and I believe my problem is at Mettaton, where I killed him too early. It says I have "34 left" but lets me proceed to the "New Home" anyways. Is there a way to go back, or do I have to start completely over?

(Note: I also know how to change the game's files, but I have no idea what all the row's values mean. A link to a guide would be GREAT.)

Solution 1:

If you didn't keep your save from before killing Mettaton, your Genocide route has been aborted.

On a positive note, though: you're currently on track for the Alphys ending. The only way to get this ending is to kill Mettaton early in Genocide, so if you're planning to get all the endings, you might as well finish this run and retry Genocide next time.

The Undertale wiki has some stuff about save editing, but I suggest you approach it with extreme caution, as you could completely corrupt your save file.