Was the goal to outright kill him or would capturing him, for some reason, have been more beneficial? Simply killing a person for the sake of killing a person does absolutely nothing, in many cases, I'd say.
I'm sure they'd have liked a capture if the option came up, but they weren't going to risk a missed chance by hesitating to shoot. It's true that his death in itself is unlikely to do much; I doubt that he alone held all of his organization's secrets in his head, and no no one will know what to do without him. There are other leaders that can pick up the slack. However, he was an icon of what he stood for, and the death of an icon can have a considerable impact. Those who had considered joining may lose some faith in the group, may be afraid to step in line with a force that just lost its head, and current members may have their loyalty shaken, especially during the subsequent disorganization.
For the most part though, yes, it's more about people declaring victory over an enemy. That's the way it's always worked; leaders take much of the credit, good or bad, often regardless of their actual influence. An act creates an enemy, an enemy needs a name, a named enemy becomes the target for those wronged. It may or may not be of any benefit, but people will always seek that goal and celebrate its victory.
Edit: The death of an icon can also empower a cause as a martyr, of course, but that rarely stops people.