"Hey what happened to Gary, he was our best security guard?"
"Oh Gary? Worked here 20 years? Was a gentle giant? Customers loved him, how friendly and diligent he was? So helpful and welcoming? All the other employees loved him, he always covered other peoples shifts, and also helped defuse work and customer issues and drama and all round lovely guy? Well there was a robbery the other day, 300 armed men with automatic guns, Gary got shot after he stopped 299 of them, and jumped in front of a storm of bullets that would have hit a family of children. He saved so many lives that day... for some reason a thousand people were in the bank and he saved them all..."
"Oh... Gary died?"
"No we fired him, he didn't stop the bank robbery... so... "
Depends. If the security guards did everything they were supposed to according to protocol and training and the bank was still somehow successfully robbed, then no they should not. Nobody is perfect, and the guards did everything within their power but sometimes it's not enough. Does that mean they should lose their jobs? Of course not. They deserve a second chance and if they prevent any lives from being lost, then that's what truly matters.
Now if the security guards were negligent and careless in their duties and the robbers had no plan going in and somehow robbed the bank, then the guards should probably lose their jobs because had they done their part, who knows? Maybe the robbery would've been foiled or not as successful. Also, if someone dies in the process, then that blood is on the hands of the security guard(s) for not following their daily protocol.