But if for the nature of the hostilities an armed combatant cannot distinguish himself from the civilian population, the people defending themselves from him can't be expected to distinguish him from the civilian population either.
And if the armed combatant has the complete support of the civilian population, if they are themselves civilians and the civilians voluntarily help them and hide them and are them, at some point you have to recognize that the people themselves are the enemy.
We have become accustomed to the rather sanitary fantasy of a war in which all you have to do is topple the enemy government, break their toys, and liberate the common people from the dictator's yoke and the war is won. This ignores the fact that the more common kind of warfare goes much deeper, it is a war between people, between cultures, and overthrowing the generalissimo isn't going to make any difference. There are times when the only road to peace is to expel the enemy and his people to a safe distance.
Hamas understands this, so do most arabs. They don't want to defeat Israel's government, they want to drive Israel into the sea, they want to drive them out of the region altogether.
Israel needs to understand that Hamas, and the people who are behind them, are not going to surrender and become real estate salesmen. They have to be driven out.
He can and must, he chooses not to. Combatants are required to wear a uniform or distinguishing markings, to carry their arms openly, and not to mingle with the civilian populatin. And he's a legitimate target when mingling with the population, even in a school or hospital, the blame for resulting civilian casualties being that of the illegal combatant. Not that anyone cares.