That ignores that Hamas is, messed up as it is, one of the few things preventing Israel from turning Gaza into a wasteland and then Lebensraum (which they're already doing, but you can probably imagine that it would be going a lot faster if there was nobody to resist). You take away however many of Hamas's numbers you can provably convict for war crimes (I'd wager the number is a lot less than most would expect, but that aside), and Palestinian resistance suffers greatly as a result. Israel takes the opportunity to oppress Palestinians even more, and those Palestinians are rightly convinced that the international community is an enemy just like Israel, only hiding behind a layer of hypocrisy that they call justice.
The only environment where war criminals in Hamas can be convicted is after Palestine has its own state, and with Israel in tow. Selective justice is oppression, not justice.
Better nothing than worse than nothing. There's a reason the ICC is criticized as a neocolonialist tool for the West and the last thing anyone should be doing is prove that right.