Cast Lead, Amnesty's discussion about Israel's claim Hamas was using civilians as human shields:
"4.2.2 ISRAELI ALLEGATIONS ABOUT USE OF HUMAN SHIELDS
"The allegation that Hamas was using human shields was repeatedly made by Israeli
government and army officials and spokespersons. In response to any questions about attacks by their own forces against Palestinian civilians or civilian objects or about the impact of their forces operations on the civilian population of Gaza, Israeli officials invariably responded that Hamas must be held responsible for any harm caused to civilians by Israeli attacks."
"Amnesty International, for its part, did not find evidence that Hamas or other Palestinian
groups violated the laws of war to the extent repeatedly alleged by Israel. In particular, it
found no evidence that Hamas or other fighters directed the movement of civilians to shield
military objectives from attacks. By contrast, Amnesty International did find that Israeli
forces on several occasions during Operation Cast Lead forced Palestinian civilians to serve as human shields. In any event, international humanitarian law makes clear that use of human shields by one party does not release the attacking party from its legal obligations with respect to civilians."
http://www.hybridstates.com/wp-content/uploads/2009/10/mde150152009en.pdf
Two points about this, first, in Cast Lead, there was no finding Palestinians used civilians as human shields. Second, even if they had, Israel still owes the very same obligations to civilians as it had if civilians had not been used as human shields. This means Israel cannot directly target civilians and civilian objects, whether or not Hamas uses civilians as human shields. And Israel cannot engage in disproportionate or indiscriminate attacks, whether Hamas uses civilians as human shields or not. Israel's obligations to abide by The Fourth Geneva Convention do not change.
Conceivably, you can have a situation where a civilian shield is killed. Btselem addressed an incident where a man Israel was using as a human shield was killed. Israel commits a war crime by using that civilian as a human shield. And if we had evidence the Palestinians who killed the human shield knew the man was a Palestinian civilian, they would be committing a war crime, too.
Conclusion, commissions of war crimes by one side do not operate to relieve the other side from abiding by provisions of The Fourth Geneva Convention.
Sherri