United Nations Security Council (UNSC) resolutions and International treaties define crimes against humanity as the "participation in, and knowledge of, a widespread or systematic attack against a civilian population, and the multiple commissions of [such] acts ... against any civilian population."
These international binding instruments go-on to impose uniform mandatory counter-terrorist obligations on all states to "bring to justice the perpetrators, organizers, and sponsors of terrorist attacks and stresses that those responsible for aiding, supporting or harboring the perpetrators, organizers and sponsors of these acts will be held accountable.
There is no escape clause - UNSC repeats "its unequivocal condemnation of all acts, methods and practices of terrorism as criminal and unjustifiable, regardless of their motivation, in all their forms and manifestations, wherever and by whomever committed." Terrorist organizations such as Hamas that kidnap Israelis, blow up and destroy public buses, murder young people at cafés and discos, and school outings, alongside indiscriminate around-the-clock shelling of hundreds of Katyusha, Qassam, and Grad rockets targeting civilian populations in Israel, are clearly committing Crimes Against Humanity - acts of terrorism for which the UNSC under Chapter VII of the UN Charter directs everyone to fight terrorism by all means.
International law leaves no room to question Israel's right to defend its citizens against systematic and sustained terrorist attacks launched from Gaza, by Hamas. Israel's reaction to aggression is nothing more than a measured, fair response designed to effectively terminate the attack [s] upon it by terrorists supported by Iran, Syria, and Lebanon, in order to prevent its recurrence. Israel is far from using all means in fighting terrorism, as required by UNSC.