Shinji just does not abandon. No matter how foolish or desperate the situation, he won't compromise on that. Especially when they are an ally who saved his life multiple times and helped him getting closer to his objective. And we all know how Shinji is when it comes to Sion.his senpaia girl in need for help
How many times could she have just abandoned him to die alone like a dog? Yet, she not only has been shown to attend his wounds as best she could while he was unconscious more than once, she even went the extra mile of providing him with necessary tools to have better odds, as well as offering advice and support until that point.
It would go against Shinji's very own character development to accept a deal with that guy. Whether death awaits or not, is irrelevant. No matter how infinitesimal the chances are,the moral obligation to curb stomp that guy here and now.we havehe has
Plus, we can agree that what was bound within the Isle should stay there for a reason.