That's probably the most rational course of action at that point. But that's not what was being decided, the question was 'did he act unreasonably?'. Unreasonable and irrational are two different things, and in this case the jury decided - taking into account all the events leading up to the killing - that trying to take the keys while keeping the gun in one hand was reasonable. So in the context of having 5 extremely drunk people (Boushie's bac was 0.3 which is high enough to induce organ failure in an average person) trespass on his property, with intent to steal, clearly posing an unusually high level of threat to Stanley and his family, he wound up shooting one of them in the head (unintentionally, according to himself). In a situation like that, the average person can't be expected to think clearly or rationally. Their adrenaline is through the roof and they're reacting to severely negative things they may never have experienced before in their life. At the end of all of that, trying to pry the keys out of a truck full of drunks probably isn't rational, but it's not entirely unreasonable given the circumstances that lead up to the event.