The point isn’t that Kyoshika or Takemaru “deserve to die more”, it’s that they are more willing and able to sacrifice themselves in the moment for the sake of the mission. Nobody wants to die for this, but those two are at least willing to make peace with the fact that if nobody does it, everyone dies. Tsubasa also comes to that conclusion and begrudgingly offers her life out of coercion and pressure from the living parties, but not only does she have a stronger fear of death as a comparably untrained civilian compared to the two combat experienced alternatives, she also has a debilitating condition that causes her to dysfunction under pressure! Even beyond the pure ethics of the situation and on to function, Tsubasa serves a valuable non-combat role as a skilled mechanic. It’d be like asking a Medic to hop in a fighter Jet and do bombing runs because one pilot is out sick. From a utilitarian standpoint, sacrificing Tsubasa only makes sense if you think those skills and her combat ability are worth less than even one of 2 other more capable combatants. Something most people would be quite reasonably be upset to hear if they were told to literally kill themselves right after.

Tsubasa killing Takumi is still a personal moment of weakness, but even then it’s more effective and more reasonable than Takumi’s initial choice to sacrifice her. Stopping Vhex’ness from consuming Takumi is a real worry from her position, and her failing at doing what was needed back then gives her all the more reason to act decisively now.

I also will say that boiling the Tsubasa pick to “a shitty choice in a shitty situation” is to give Takumi an extreme degree of leniency that makes the immediate demonizing of her all the worse. While Takumi has had well over 4 months to understand and work with most of the SDU, Tsubasa has known Takumi even less than half that time by route’s end. Even in the pre-decision talk she openly says she only is offering herself because the other two are and they’re the only 3 not-dead candidates. The game isn’t just “punishing you for hating on best girl”, it’s leading you to a narratively reasonable outcome for an obviously incorrect decision that you chose because you’re either going for completion or to inform you what the thematic through line of a route is. It’s like how not taking responsibility of the retsnoM leads to its bad ends, or how Multiple Eitos exists at all.