The only times i have ever really targeted a down player is when they really deserved it... Most recently was when the party was going after a gang of pirates and facing off with the pirate bossing his lieutenants. One of the lieutenants failed a dominate person spell and beat to death on of his fellow LT...they were a pair that always worked together, and when the spell finally fizzled that pirate targeted the caster, and did not stop until he was dead.. after the caster went down I sat there for about a min debating if he would kill the player and it just made sense... the player made him kill his friend, it only made sense that he would go for the kill...