IMHO, this debate is not really over whether the orc would or would not have a logical reason to smite the fallen cleric (we could come up with dozens, as I think we already have), but it is really a question of whether the DM should have purposely killed off the PC.
As has been already stated, the answer to this question depends greatly on the attitude/maturity of the people at the table, as well as the DM's personal opinion on what constitutes "fun."
Personally, as a sadistic DM of diabolical proportions, I make it no secret to my players that I am out to kill them. Of course, I never...well rarely...um...on occasion have been known to dump them into deadly situations rather beyond their capabilities. But they always have the option of fleeing (or avoiding the situation altogther if they haven't been careless). But once they engage in combat, I AM OUT TO KILL THEM. If there is any logical reason for a creature's lethal behavior, I use it. I stick to the die rolls, stick to the rules, and stick to my principles (my chief principle being that I get five more points on my Evil DM scorecard if I get a TPK).
Like someone said: there's always raise dead.
As has been already stated, the answer to this question depends greatly on the attitude/maturity of the people at the table, as well as the DM's personal opinion on what constitutes "fun."
Personally, as a sadistic DM of diabolical proportions, I make it no secret to my players that I am out to kill them. Of course, I never...well rarely...um...on occasion have been known to dump them into deadly situations rather beyond their capabilities. But they always have the option of fleeing (or avoiding the situation altogther if they haven't been careless). But once they engage in combat, I AM OUT TO KILL THEM. If there is any logical reason for a creature's lethal behavior, I use it. I stick to the die rolls, stick to the rules, and stick to my principles (my chief principle being that I get five more points on my Evil DM scorecard if I get a TPK).

Like someone said: there's always raise dead.