#segment 45

It has everything to do with the Tatarasuna incident.

Because at 35, Zandik found and saw Kabukimono, a puppet, an inhuman being being accepted by the miners when he himself was chased out of his hometown and expelled from the Akademiya. This inhuman puppet, accepted, where as he, rejected--and envy turned into spiteful selfishness as he targeted this puppet to ruin him.

Like it was not in Dottore's mission to do anything to Kabukimono. Everything he did to Kabukimono--making him think that his human family had betrayed him in both choosing him as the sacrifice and the whole thing with Niwa, then telling Pierro about him so they could "make him an ally"--was entirely his own prerogative. His own choice. He chose to hurt Kabukimono out of his own selfish envy.

That's why I think 35 ends up being the "most selfish" segment.

Edit: I'm more convinced of this after reading the interactions between all of the known segments in the 2nd Anomalous Marrow, and how Segment 45 behaves in particular.

"The ones who need help are always the easiest to talk to." This is such a loaded statement for him to make. He is, essentially, admitting that people who are vulnerable and need help are "easy to talk to", or, more likely, easy to manipulate. Segment 25 basically spells it out for us too--that Segment 45 is being cocky because he holds all the power.

This is further implied by his views on Columbina:

Essentially Segment 45 is saying that Columbina is "approachable" because she looks vulnerable.

I think Segment 35 is the most selfish because Zandik caved into his selfish desire to ruin Kabukimono, and then Segment 45 is manipulative and preying upon vulnerable people because Zandik preyed upon a very vulnerable Kabukimono after he joined the fatui.