So my first time I played Undertale I did a neutral route. When I beat Omega Flowey I decided to kill Flowey. When I decided I wanted to do a genocide route. Flowey was gone. After that I killed every monster I came across including Toriel. I left the ruins and still no Flowey. When i got to Snowdin all of the village people were standing around and the happy music played. I think that I did something wrong?
If you also want to know that you are doing Genocide right, you have to grind until it always taunts you with "but nobody came" and the save points don't give you anything except "determination" and then a save.
The monsters will reset in every area: Ruins, Snowdin, Waterfall, and Hotlands/The Core.
I'm not sure about New Home, but I do know that the save points in all of them except The Ruins show how many monsters you have left to kill at the save points.