Well, that's a matter of perspective. Israel has been oppressing and committing war crimes on the people of Gaza for decades, not to mention the continued expansionism and murder on the west bank. If we are of the mind that retaliatory violence against civilians is justified by prior atrocities, we have no grounds on which to condemn what Hamas did, and we certainly cannot say that they cast the first stone.
Personally, I'm not a tit-for-tat kind of guy. Just because you have been wronged doesn't necessarily justify any response. The key is proportionality, intent and effect. This is why I can condemn Hamas for their barbarism in response to historical oppression while also condemning Israel's response to terrorist atrocities by committing war crimes. It's not that ANY response was unwarranted, but that these specific responses are unjustified, disproportionate and overwhelmingly negatively impact civilians.