If a DRE is available and if a suspected mental patient can be examined, the DRE can check for signs of drug use, and even if the person has mental problems, a DRE can determine if the person is under the influence of drugs like meth, coke, PCP, marijuana, LSD, certain inhalants, and opiates like heroin. (DRE = Drug Recognition Expert) If a DRE were to examine a person and found no signs of drug abuse, but the person is still exhibiting bizarre behavior, the best course of action is to take the person to a hospital and have the mental health personnel examine him. Naturally, not all officers are DREs, and not agencies have them anyway. IMO, police departments should have as many of them as they can get the training for.
Somebody with diabetes, low or high blood sugar can also be interpreted as deranged.
There is a more basic underlying problem here.
Our freedom is premised upon respect of legitimate authority.
Even LEOs must respect legitimate authority.
They are given authority by the state to use deadly force where the situation justifies its use.
Those LEOs who rebel from the legitimate authority of the state might simply live by the power of their commission, but if they also commit criminal behavior, are not above the law.