"But suppose this HAD escalated to the point where the robber killed the store clerk."
You're asking law enforcement to now predict what might happen after having received a tip to the contrary? If that's the case, its a wonder if people aren't being murdered everywhere ...
"You're asking law enforcement to now predict what might happen after having received a tip to the contrary? " What do you mean, "a tip to the contrary?" Are you saying that murder in the commision of a robbery is such a rare thing as to be negligible? Or did they receive some "tip" that guaranteed that this particular robber was harmless, and if that were the case, why didn't they nab him in the commission of the crime?