It is a trade off. He knew that his sp was low, to the point where he would have killed himeslf from his own self using any skill, so the only choice left is to shut em off and just hope not to get killed. You say why not go down fighting but if you truely know you have no chance fighting back pointlessly vs the less than 1% chance of your enemy not killing you or something happening to distract your enemy from finishing you off and surviving from that is the better chance you have that ending it for sure by activating any skill withthe last sliver of sp and ending it yourself. It is the better choice here to do so. To kumiko it would probably feel it is like she is about to win the championship and hit the last point or goal, only for her opponent to concede the last second, it would feel bitter because to suddenly see what you thought was a great strong wall suddenlycrumble before you, as you were about to accomplish breacking through or climbing over it, but robbed of the acctual moment while seconds away from accomplishing it herself, will make it feel anticlimatic to her.
