More common than you know. My Dad (Normandy veteran) told me that Americans also killed prisoners. My Uncle, also a veteran of France and Germany, confirmed it.
As a veteran of Vietnam myself I can't say I ever saw a prisoner being killed but there were certainly stories of it going around among the troops. To those who have not been there it would do well to remember the old saying -
"War is hell...but COMBAT is a M**********R"
You really have no idea.
Lots of things happen in the fog of war that are eternally lost to the mist time.
Surrendering in combat is always hazardous. Surrendering as an individual soldier is never a good idea because front line troops are typically under orders to get somewhere fast and they don't need to be slowed down by a prisoner.