Mercenaries can be summarily executed.................
“This is also laid down in Additional Protocol I, which states that anyone who is not entitled to prisoner-of-war status, and does not benefit from more favourable treatment in accordance with the Fourth Geneva Convention, still enjoys the fundamental guarantees provided for in Article 75 of Additional Protocol I.[25] Consequently, the summary execution of mercenaries is prohibited.”
https://ihl-databases.icrc.org/en/customary-ihl/v1/rule108