Pillay lambasted the country's attacks on homes, schools, hospitals and United Nations facilities which are sheltering 250,000 civilians in Gaza.
"There appears to be deliberate defiance of obligations that international law imposes on Israel," the South African told reporters.
Pillay said that repeated calls to respect the laws of war had gone unheeded during the latest crisis and previous spikes in the Israeli-Palestinian conflict.
"The same pattern of attacks is occurring now on homes, schools, hospitals, UN premises. None of this appears to me to be accidental," she said.
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Israel has accused Palestinian Islamists Hamas and other militants of hiding out among the civilian population and using UN facilities and other sites to store weapons and launch rocket attacks.
Pillay said that under international law, civilian facilities should not be attacked, but can lose their protected status if used for military purposes.
Even then, she said, due warning must be given before an attack, in order to allow civilians to be evacuated.
"It is completely unconscionable that the proportionality and precaution that international law requires is being ignored," said Pillay.
Last week, the UN Human Rights Council voted to open an inquiry into the Gaza offensive, despite fierce opposition from Israel and the United States.
"We cannot allow impunity. We cannot allow this lack of accountability to go on," Pillay said, calling into question domestic investigations by Israel into abuses.
"I join the world in condemning the aggression that is taking place in Gaza, and particularly the killing of civilians. This is wrong and it will always be wrong," she added.