What really stinks is that we really COULD have used that bullet Irene killed herself with. If Carley had had just ONE more bullet in the drug store (the one Irene used on herself), she might have been able to shoot the zombie grabbing her ankle, while Lee went to save Doug. Then we would have had both of them with us!
But I guess someone in the group had to die, in order to forge a closer bond between Lee and the one he saved (from a writing point of view, at least). Plus I'm probably over-analyzing the whole situation.