THE HAGUE — A court in the Netherlands on Friday convicted an aide to a former Ethiopian dictator and sentenced him to life in prison for war crimes carried out during Ethiopia’s “red terror” purges of the 1970s, including the execution of 75 prisoners.

The defendant, Eshetu Alemu, 63, was found guilty of ordering the 1978 killing of detainees, many of them younger than 18.

In the hearings held under Dutch universal jurisdiction, Mr. Alemu, a former regional representative under the country’s leader at the time, Mengistu Haile Mariam, was convicted of all charges brought by prosecutors, including arbitrary detention, inhumane treatment, torture and mass murder.

“The fact that the majority of victims were children younger than 18 makes the crimes all the more cruel,” said the presiding judge, Mariette Renckens.