Board Thread:Questions and Answers/@comment-37548216-20191121231547/@comment-26907577-20191121235006

Your Pacifist has no effect on your attempt at a Genocide route. Monsters progressively get rarer the more you kill them (though leaving the room resets this). The reason you don't see Genocide effects in Snowdin is that you did not reach the required kill count of 20 for the Ruins (and so, unfortunately, must reset). Again, leaving a room solves the incredibly rare encounters. As Kocka says, your 21st encounter will say "But nobody came" and thereafter spam encounters.