Sekido is getting sick of your presence you irritate him more than anything. It doesn't help that you've had to work side by side with Hantengu. Could it be he's jealous of your power and capabilities? Or that he just hates your guts? Whatever it is he despises you for it and he doesn't understand why Muzan turned you into a demon. He thinks you're too inferior to be upper rank 6. so when you two are alone while the others are fighting he waits for a moment to say something mean