Oh no, deliberately shooting dead men walking is not on. Sometimes it's not entirely clear whether someone is dead though, eh? For eg, I've been in the situation a few times where I was shooting somebody whom did not take the hits, then they move... are they taking a hit finally, or not?
Personally, if I see someone raise their hand and/or hear them shout, "Hit!", I won't shoot at them or close enough to them that an unexpected step by them will put them in my line of fire. However, if I'm not sure, I will shoot, just less aggressively than I would against a known live target.
Then there's the people who wander around at the periphery of a skirmish - are they non-combatants or players who are nonchalantly seeking to flank us in the, mistaken as it happens, belief that they are unseen and/or out of range? Again, if I'm unsure I aim low and make my first shot or two sort of 'exploratory'. That said though, at the GZ Weekender I got hit about four times through being unsure and not sufficiently aggressive.
I'd like to go with a zero tolerance policy of shooting anyone who isn't obviously drawing attention to the fact that they are non-combatants. If I must cross people's line of fire I do hold up a hand and shout, "Dead man walking!", or something like that and if people shoot me whilst I am doing so, I vociferously question their intelligence, parentage, and, occasionally, reason for living. The trouble is that regardless of what I think, when it comes down to pulling the trigger, if I feel unsure I just can't go for it...
Here's one for discussion mind: how about shooting dead players whom are being used as cover by live team mates?