Making Shinichi kill someone is such an interesting concept. The boy who believes killing is abhorrent, who will go out of his way to save lives many wouldn't or have seen as too far gone, having to make the choice to kill because if he didn't it would mean someone else would die and exploring how he lives with it.
Even if it's Gin he has to kill to protect Ai, Shinichi would struggle so much with the choice he made and blame himself for putting them in a situation where that was the only choice he could make to rescue her.
God and if it was to protect Ai, that would be a horrible situation for her too because she is one of the few characters who know Shinichi the best and knows that what he did would be something he would struggle to handle and yet she would think "because of her I put him in such a bad situation again" and the duo with suffer with their own guilt over it.
Making Shinichi kill isn't something you should do lightly but BOY is it a fun scenario to explore.