You'll see them dispatch mortality wounded Russians often, presumably partly to end the suffering. That guy was already dying, there's no hope of getting him care.
Maybe there was no hope, but that's for medical personnel to decide. Mercy killings were a Middle Ages thing, nowadays once an enemy is no longer a threat, whether by being wounded, surrendering, getting captured, or in any other way, then it becomes a war crime to kill them.
This kind of videos play straight into Russian propaganda.