You're raising an important existential question.
Suppose there is a war between a hypothetical Force A and Force B.
Force A is proud of its concern for all life; and further proclaims its military to be the best trained and most moral on Earth.
Force B thinks it has an advantage; believing Force A would not harm children they store their weapons in a grade school.
Force A learns of this and using its superior weaponry destroys the school, killing a dozen children and maiming dozens more.
Who is the monster?
My father was a machine gunner in the Korean War. On occasion, when the Chinese troops attacked, they would march the residents of the local villages in front f the first wave of troops. Dad said simply, "you tried to miss the civilians". Sometimes, there is no neat and clean option, a position troops are put in by their enemies who put the civilians in harm's way. The attackers must be dealt with, the civilians are "collateral damage" which sucks for them but war is especially Hell for the civilians, anyway.
The monsters are those who use the civilians as human shields, or attempt to.