Scott Roeder specifically said that he killed Tiller in order to prevent him from killing more babies in the future, not to avenge babies he killed in the past. And there was certainly a reasonable probability that Tiller would, indeed, have continued to kill more babies. This is not to say that the shooting was justified, but that the motive was to prevent future abortions, not past ones.
Somehow, the abortion industry managed to replace the irreplacable Dr. Tiller, but that wasn't really my point. Obviously, you kill Hitler because you're hoping to end the war/holocaust. If you know that Himmler will fill Hitler's shoes comfortably, there isn't much point in killing Hitler, other than to punish him. Same thing goes if there are other doctors willing to perform abortions. Admittedly Tiller was the worst of the worst, but others remained. How many lives did he really save? And the thing is, Roeder seems intelligent enough to realize this. No matter what he says, I still think he did it in part because he appointed himself executioner.