What are you on about, what are you asking? why are you asking it?

This is false dichotomy because the intentions of the one using the human shield is known. On the other hand, the one who attacks the human shield needs plenty of context to determine his intentions, motives and decision making. They may not know the human shield is a human shield unless it is obvious or they are being alerted on the facts. Also, they may need to attack as there is clearly something off about the situation that they will fight to survive because there are times where you are at a disadvantage or terrible desicion making put you in a bad spot. Lastly human shields are not always tied up and may act independently enough that they are consciously accepting someone's orders as a volunteer. There could be stray shots or the human shield intentionally gets in the way. There is so much context required that the "why" is not always subjective. It can be objective reasoning or random sequence of events that were badly done.

Not discounting the fact that there are situations where the "why" is subjective, like you are talking about where the person knows about the human shield and intentionally just targets them for no other reason and it is not collateral. However, this is a sterilized scenario that does not always occur.