A state crime is one that is undertaken by a government and that breaks the laws of that nation in terms of the common laws that they have in place.
Common examples of state crimes include war and genocide. The main condition for a crime to be considered a state crime is if it were done in the state, which would already be subject to laws that prohibit the act.
State crimes are often said to be committed by the leaders of the country that enacts the crime. Those leaders are subject to be held accountable in the International Court in the Hague.