The difference between a terrorist attack and genocide is the power balance involved. A terrorist organization like Hamas, similar to the Provo IRA in Northern Ireland, has no real (no legitimate) power compared to the occupying nation (Israel in the former, Great Britain in the latter) and therefore their terrorist acts cannot be considered genocide--it's mass murder, to be certain, but in retaliation for oppression; as JFK is purported to have said, "Those who make peaceful revolution impossible will make violent revolution inevitable." That is in no way an endorsement of the murders, just an admission of their inevitability in a state of oppression. Genocide, on the other hand, is a systemic (usually government sanctioned) attempt to remove a people from a land, not just through murder, war, and starvation, but displacement, turning them into a diaspora (a people scattered across nations, as the Jewish people were for centuries); whether Hamas would want to commit genocide is irrelevant, they do not have the means to do so; the power dynamic in the region is too heavily weighted on Israel's side. The Israeli government DOES have the means, and their actions prove that to be their will; as to who fired the first volley, prior to the attack, the Israeli government had already been engaging in forced evictions of Palestinians, giving their homes to Israeli (and Jewish diaspora) settlers, an attempt to displace the Palestinian population. They see all Palestinians as a threat, not just Hamas, and over 40000 Palestinians are dead because a handful of them killed less than 1200--a grotesque loss of life amplified by a grossly disproportionate response, similar to our own war on terror, the longest armed conflict in US history.
But to be fair, none of that is even the point; again, even if Hamas (a small percentage of the Palestinians in Gaza) were trying to commit a genocide, even if Hamas were using human shields, the Israeli government is still responsible for the deaths of 14,000 children, and that is indefensible.