There's very little you can practically do in these cases of extreme grievance, other than avoiding putting fuel on the fire.
This is really partly why companies have a system of sick leave, to avoid sacking the "unstable".
A restraining order does not practically alter the situation (it's more suitable when someone is repeatedly visiting to worry or harass the staff, rather than someone who is threatening to perhaps turn up once by surprise and execute the staff).
Police action itself may trigger the undesired response, either by adding aggravation or by unbalancing the person's life further (i.e. further undermining the controls on their behaviour, rather than adding additional controls).
You might find that the police response is informed by their experience in this regard - that people often mouth off, and if things are left alone for a certain period of time and nothing has occured, and there is no further provocation, then nothing further is likely to happen.
Whereas if he is seriously capable of proceeding to a murder in these circumstances - and that is the unclear part - then even jailing him for the threat is as likely to tip the balance and produce someone who will come out and murder those who (in his eyes) put him there and ruined his life.
Moreover, those who are capable of such conduct are themselves capable of acting strategically, and if you make it impossible for them to issue threats that would alert people that a dangerous situation is developing (by severely criminalising the threat alone) and which might allow the targets to desist from anything that might further provoke, then the attacker can respond by remaining silent until actually striking without giving warning. People getting killed is usually considered a far worse outcome than people getting frightened.
So again, the police may be reluctant to act on mere threats, because of the potential to further induce behaviours they are trying to avoid.
And it doesn't matter how much extra security you put around the building - people have to come out some time.
Finally, if he is a former police officer and especially if you're in a less developed country, you might find that there is a certain amount of lenience from ex-colleagues - especially if (for better or worse) they are familiar with his character or circumstances.