When are prisoners of war allowed to be executed?

Update: The prisoners were released in a deal with Saudi Arabia
https://youtube.com/shorts/pkSeYxemQxY?feature=share

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The names of the condemned are:
Aiden Aslin and Shaun Pinner (Britain) and Brahim Saadoun (Moroccan)

By my research, it seems like the British men were a part of regular Ukrainian forces. There isn't as much information about Brahim Saadoun, but some news agencies have said that he is a student.

At least one man, Pinner, has a Ukrainian spouse.

References:

3 sentenced to death for fighting on Ukraine’s side
https://apnews.com/article/russia-ukraine-moscow-business-blockades-74883ed750fb8778259cf733e1095fb8

Separatist Court In Ukraine Sentences Three Foreign Fighters To Death
https://www.rferl.org/a/ukraine-separatists-death-sentence-foreign-fighters/31891033.html

Summary of the Geneva Conventions of 1949
and Their Additional Protocols

https://www.redcross.org/content/dam/redcross/atg/PDF_s/International_Services/International_Humanitarian_Law/IHL_SummaryGenevaConv.pdf