This strikes me as a bizarre argument.
Let's say we have two populations, A and B.
If population A is committing a vast number of casualties compared to population B, why is it important to note that there's a segment of population B that wants to kill every man, woman, and child in population A, even if we assume that's true?
There are surely members of population A who feel exactly the same about population B. But even if there weren't, isn't the number of casualties more important than how much hatred exists on each respective side, ESPECIALLY considering that Israel doesn't use guided missiles that fly directly to members of Hamas and kill only them, rather than destroying Palestinian homes generally and murdering 60 children among the 200 killed in that particular attack?
If some of my family hate your family's guts, and let's say my uncle picked a fight with your uncle and ended up killing him, in what sense would it not be an overreaction for you to show up and kill nearly all of us?
Why wouldn't it make more sense to show Israelis performing air strikes on Palestinians with bombs marked "death to Arabs" and saying "Palestine is attacking us-- this is self-defense"?
Oh, right. Because Palestine didn't attack them.