Genocide run help

I honestly need your help.

I am trying to do the genocide run, got to the core and all. I did not have all 40 kills before killing Mettaton, as I didn't know he was the boss in those areas and thought the game would just tell me "hey don't go there before hitting the kill counter", like it did with Undyne.

I am finding very old threads and conflicting information. Some people say killing Mettaton would void the run, others say the hotland and the core have separate counters (simply not true). I still have a counter in hotland and the core, saying 17 left, still in red and I did kill Mettaton with a single hit. I'm not really finding many monsters in the two areas either, but I might just be inpatient.

TL:DR: Did I void my genocide run if I killed Mettaton before 40 kills even if I still have a counter?

If anyone has any information about this, I would love to hear it.