Mouseferatu
Hero
That's the surest way to lose your players. You don't have monsters attack downed PC's to kill them. The gnolls can stuff their extra attack!
Depends on circumstance. Many villains/monsters will prefer to focus on the people who are still a threat, absolutely. But some--either out of cruelty/malice, out of the knowledge that a dangerous enemy could still be revived, out of simple hunger, or because they can't reach another combatant anyway--absolutely would take the time to smack a downed foe.
If a DM does that often, it's a problem, but occasionally and when it's honestly appropriate? I'd not only be okay with him doing so, I'd expect it. And--again, if done under proper circumstances--I'd have a serious problem with any player who got upset enough to threaten to leave a game over it.