The genocide route is the exact same as it was before, the "update" was just one change: Making the final boss actually impossible without cheating.
It's the only morally correct route. Kazuya just goes off the deep end far too fast. Genuinely though, if you think any of the demons should be spared (besides Mukuro and Sphinx, they're the only good ones), please give your reasoning as to why.
I didn't played the genocide route, I don't know what happens in it.
But when I play a game, I don't play with "me". In Shrift, I play Kazuya and I try to make the choice Kazuya would do because he is a kind man. I don't think Kazuya would go berserk and kill everything. Maybe he'll kill some monster, but not everyone, trying to purify every places. The pure genocide route is the opposite of Kazuya. He just can't be like that.
But if you play and want to make the best moral choices, I think you are right, every demons should just die. They deserve it. And it's always self defence for ourself and the human kind.