How do companies protect themselves from disgruntled workers that are fired ?

An old boss of mine said he would only fire people at the end of the day on a Friday. The reason-> they had 2 days to process their emotions and less likely to do anything stupid.

better security when employees are entering a building

There is nothing you can do to completely nullify the risk

Automatic killer drones

as was being discussed on another post, ethically speaking the fault lies with an incompetent justice system. The shooter was a felon with multiple priors; stabbed a previous girlfriend, made death threats to others, multiple assaults, and only served 3 years in prison for it...

Protecting from what?

stop making work so boring that they end up being that 🤔

Lock up all men.

2nd amendment.

Kevlar.

Usually the police actually confiscate weapons from those who should not have them and then that prevents bad people from walking around with guns and shooting up their workplace when they get fired because: they were not a good guy.

You can't. It can happen anywhere even with armed security.