I hate to say that, but killing a unharmed, fleeing soldier is a war crime.
Well, according to ICRC website, fleeing is a reason for loss of protection, so no, this is a legit use of explosive. Did he attempt to surrender, he could have survived.
https://ihl-databases.icrc.org/en/customary-ihl/v1/rule47#title-6
You can get a phone number for a little more than a dollar a month at OVH. You can just redirect all call to voicemail and check once or twice a month and call it the day.