Karasu just positioned himself to potentially get subbed off for Sae in the future.

I think he might have doomed himself right there in that moment.
And there might be signs for his ego as well but lets see where the story goes.

Its like if Isagi was the one to sacrificed himself to become the best #2, he instead would be a huge candidate to get replaced for Sae.

Thats just what I feel like its going to happen so far.