Can you give an example of a time when a "legitimate military target" needed to be killed so immediately that a unilateral presidential decision that also killed nearby citizens was warranted? My presumption is that there are very few circumstances where the threat of a military target is so immediate that killing nearby innocent civilians is justified over waiting to try to find a better time to attack the target. If anything, immediate threats that require action are generally threats to the nearby innocent civilians themselves, which would make such decisive action not particularly useful; I don't think anyone would suggest something like blowing up a bank where people are being held hostage, which seems like the closest hypothetical I can think of.
I would argue that the US law would not allow the president to order strikes like those that have occurred in Gaza on US citizens, but of course IANAL. I don't have any knowledge of what Israeli law allows or doesn't allow though.
It's also worth noting that what someone thinks is legal is not necessarily what someone thinks _should_ be legal; I imagine that almost everyone here has at least some laws that they disagree with, but that's separate from the question of whether the law exists or not.
Enemies are not static they can react to our policies. If our policies are never to kill anyone who is near a civilian, enemies would just always be near civilians.
