It is a crime to kill Daesh terrorists without the permission of European governments.

Alleged photo of the arrested Dutch national
It is reported on Sunday that an ex-soldier of Dutch military is arrested on a suspicion of killing ISIS/Daesh terrorists in Syria.
The Unnamed, 47-year-old ex-soldier was released later. The authority made him surrender his passport as well to stop his further trip to Syria.
The prosecutors said in a statement, “Killing an IS fighter therefore could mean being prosecuted for murder.”
Netherlands is part of US led coalition against the ISIS. The prosecutors also said that there was “an important difference between Dutch nationals who travel to Syria on their own to fight against IS and Dutch soldiers who train Iraqi and Kurdish forces”. The statement added that the Dutch military presence was legal and took place “at the request of the Iraqi government”.
Prosecutors suggested that further investigation into the soldier’s involvement was required to decide whether charges would be brought against him.
According to some western reports more than 100 Western volunteers have been fighting with the Kurdish forces in the region even though they knew there will be legal troubles waiting in their countries on their return.