https://www.icc-cpi.int/sites/default/files/Publications/Ele...
Israel has been killing Palestinian civilians. Israel has been restricting food and medical supplies going to Palestinian civilians. All of that was beyond reasonable in regard to military objectives (e.g., bombing hospitals to take out a weapons stash). Israel's political and military leaders have expressed their opinions that Palestinians (not just Hamas or militants) are an ethnicity deserving punishment.
Note that, in the ICC's definitions, genocide does not require intent to eliminate the entirety of a group. It only requires targeting people based on their belonging to a protected group with the intent to diminish that group.