But it was a mistake born out of a panic reaction.
For minutes Shinji had been trying to keep Asuka from skewering them in a fully one-sided fight because he refused to fight back. First Asuka destroyed 2 of their 4 AT Field generator drones. Shinji asked for Kaworu's help, but he continued mumbling without even looking at him. Then Asuka destroyed their remaining two drones, and Shinji again begged for Kaworu's help who again refused to even acknowledge him. Then Shinji had to fend off Asuka's attacks without any AT Field or weapon of their own, and they only survived this one because 02's battery ran dry. Effectively, the minute while Asuka was changing batteries was his only window of opportunity to act and not get killed. And Kaworu ignored Shinji's pleas twice, and was clearly out of it. A mistake is still a mistake, but context is important.