One thing we learn even in situations that are not in the Middle East is that actions cause incentives.

Refusing to kill militants when protected by human shields encourages the taking of more human shields.

In any case, international law, and I would say common morality as well, condemns the killing of civilians if it doesn't accomplish a reasonable and "proportionate" military objective.

It's not hard to bring up the Wikipedia article. Proportionality requires that the force be proportionate to what is necessary to achieve the objective, not that the force be proportionate to the number of deaths caused by the enemy.

for each dead militant, Hamas will be able to recruit three from the enraged population.... This combination of large loss of civilian lives and minimal accomplishment of military objectives, particularly given the status of Gaza as an occupied territory, are the reason why people argue that Israel actions are disproportionate and doubt Israel good faith in its claims that it is trying to minimize civilian losses.

If this was truly what they believed, then I could, for instance, argue that Israeli attacks don't make it easier for Hamas to recruit from the enraged population, and it would also count as an argument that Israel's attacks are proportionate. I don't believe for one second that such an argument would be received in such a way.