Ok...say there was no threat or expressed intent to do violence, but it was noted that he was very disgruntled.
The police are watching him and he purchases several weapons and ammunition. Still no laws broken.
So, being reasonable, the police decide to continue surveillance of the fellow.
The next day he loads the car with his new weapons and ammo and heads off towards his former place of employment. Still no laws broken.
The police, being suspicious have officers on hand a the employer's place waiting.
The guy arrives, takes a few deep breaths and gets out of the car, grabs the weapons and fills his pockets with ammo and heads towards the front door. It is likely there are still no laws being broken.
He won't be breaking any laws unless someone tells him to get off the property.
At some point before he draws down on someone, it might be wise to stop him.
At what point are the police not going to violate his rights by detaining him?