To summarize and expand a bit, Yerushalmi Terumah 8:4 says:
- If travelers are accosted by a group of gentiles who say "give us one of you or you'll all be killed", they can't turn someone over.
- If the murderers designated somebody, as in the case of Sheva ben Bichri, then they should turn him over.
- R' Shimon ben Lakish says: only if he has incurred the death penalty, like Sheva ben Bichri had.
- R' Yochanan says: no, we can hand him over even if he hasn't incurred the death penalty.
An implication of the first two bullets is: guilt for murder falls (at least partially) on the one who does the choosing. If the murderers choose somebody then we can hand him over to save the group, according to R' Yochanan. Reish Lakish places additional restrictions but still allows it in some cases -- so we're allowed to hand him over if we didn't choose him.
But, you argue, there's a special case of a "dead man walking", somebody who's dead anyway but it just hasn't been carried out yet, so why can't we extend that to choosing that guy specifically?
The problem isn't that he's subject to the death penalty (or not); it's that we would be doing the choosing. There is no "life-value math" that allows a group to sacrifice one person to save the rest; all human life is infinitely valuable to HaKadosh Baruch Hu. (R' Jonathan Sacks makes this argument in "The Practical Implications of Infinity" in To Touch the Divine, as cited in some study materials I have.) In the specific case where the murderers choose, we are allowed to go along with that and let the guilt fall on them, but we cannot take an active part in choosing who will be killed, because your blood isn't redder than his.
We can make a further argument based on breaking objects. In Bava Kama 17b we learn that if somebody throws another's (clay) vessel off a roof and, while it's in mid-air, somebody else hits it with a stick and shatters it, that second person isn't liable because it was going to break anyway. But the tosafot there says that if, instead, somebody threw a rock at the vessel and the second person hit and shattered it, the one who actually broke it is liable -- because the rock might not have hit it or might have hit with too little force to shatter it. So in the one case the vessel is "already broken", but in the other it is not. I would argue that a person subject to the death penalty but who has not yet been executed is like the second case -- a witness might still come forward to exhonorate him or the court might not carry out the sentence for some other reason. So, while the person is under a sentence of death, while it is likely that he will be killed, it's not certain. The death sentence is like the rock being thrown at the vessel, not like the drop from the roof. (This is my own argument, but I wouldn't have been able to make it if I hadn't taken a course last year where these passages came up in two different sessions, the first on trolley-car dilemmas and the second on hastening inevitable outcomes.)