Well said, Lo Pellegrino. 

I did once evict a customer when he exchanged words with one of my employees, which included homophobic slurs (the employee in question is quite openly gay) and threats of violence. I honestly did try to reason with this gentleman, but he decided to get cute and push me, so I put him in an armlock and marched him out the door with help from one of my other employees. We took his photo and informed him that if he came back it'd become a police matter. Haven't seen him since, thankfully.

The reason this guy decided to pick a fight? We accidentally gave him the wrong drink.

Some people just have a chip on their shoulder, and think that it's okay to abuse people who they think don't matter, for instance wait staff at a restaurant. They're wrong, and you have every right to be treated with courtesy and respect. That person should not have threatened you over something inconsequential.