Using indiscriminate weapons in areas where civilians are known to be present in large numbers is, indeed, illegal. International law is itself problematic, as are most legal systems. But that's what it is. It's not what what you, I or anyone wants it to be, it is what it is. The use, for example, of white phosphorous, which Israel used in gaza in 2009, is illegal in civilian areas and is proscribed for specific uses in a field of battle--something rarely seen in any war these days. But it is not illegal altogether, only if used improperly. If, for example, Hamas was able, with some degree of certainty, to aim a rocket at Israel's ships off the coast, or at Israeli troops massed at the gaza border, the use of indiscriminate weapons would be permitted since it is not being used in civilian areas. Another example is a bomb. A suicide bomber, say like the one Hezbollah employed at the Marine barracks in Beirut in 1983, is not an indiscriminate weapon. It can be used in such a way as to target combatants and locales used for military purposes. In any case, Hamas makes no bones at all about the fact that it targets civilians with its rockets, so the point is moot. Whether a weapon is indiscriminate or not, willfully targeting civilians is a war crime.

On the genocide question, what makes it different is that Israel is not doing what you quoted, plain and simple. The fact of murder, mass murder, is not the same as genocide. The United States killed a great many more Iraqis than Israel has Palestinians, that doesn't make the US actions aimed at wiping out Iraqis, which is the intent and point of the genocide statute. On the other hand, the object of the hutus in Rwanda was to wipe out the Tutsis. That's the difference. Again, it may not be how you would define these terms. But this is what the international legal system has established as its definition. Israel doesn't fit it, nor do we need it to--what it is doing is quite horrific enough. Again, the law may not say what you or I might want it to say, but it is what it is. Progressives often believe that International Law is some trove of wisdom, when it was made up initially by military leaders, and later on by lawyers. Expect not perfection from those groups.