Because they were fighting for their lives. Against Muslim Palestinians who where murdering their women and children in an attempt to wipe all the Jewish Palestinians off the map.. Why do so many people fail to understand that the Jews and the Muslims that lived in the area that is now Israel are BOTH Palestinians? Look up the "Mandate of Palestine" and get educated.
U.S., allies stage 20 strikes against Islamic State in Iraq (Reuters)
from the article: Hamas military chief Ahmed Jabari killed by Israeli strike