First, you are assuming the issue under question. Yeah, stabbing someone over being asked to wear a mask seems crazy, but so does parading into a Target en masse chanting that “masks are killing people, take off that mask,” or refusing to leave a football game when told to wear a mask or leave, and not only forcing the police officer to actually arrest you for trespassing, but resisting such that he has to taser you to get you to comply. Plenty of things seem crazy, but aren’t actual mental illness.
Second, a lot of people “go around with a knife” - I carry one almost all the time. A pocket knife is a useful tool, and it can be used for self-defense in an emergency. It also could be used as a weapon of aggression, but merely carrying a knife does not show an intent to use it on people. I’ve carried a pocket knife “literally” since middle school, and never used it on anybody, self-defense or offense.
Whether one should cheer over someone else getting shot or dying, well, some people have less tolerance for violent assholes and are more appeased by karma.