Those hundreds of rockets kill very, very few people, as they are very primitive. Of course every death is a tragedy, and a war crime, but for example in the last Gaza war those rockets killed I think 5 Israelis, whereas the Israeli counter-response killed over 1,000 Palestinians.
If Israel wants to stop the rocket attacks, they need to think strategically and long-term, not just tactically and short-term. That means making a serious effort to resolve the conflict politically. There are reports that a long-term truce with Hamas had been in the works when this all started up.
Israel's response here has nothing to do with military necessity and everything to do with the upcoming Israeli elections, the ongoing confrontation with Iran, and Israeli insecurity thanks to all the Arab Spring instability. All my Israeli friends say "Of course Bibi has started a new conflict, there's elections coming up."
Anyway, the OP was about war crimes. Israel may well find its offensive neutralises the rocket attacks, but that doesn't mean it doesn't violate international law.