People are crazy! There's no reason to harm another person just because they got their order incorrect. I thankfully never dealt with someone this terrible when I worked in customer service, but I've had my share of bad customers. I heard that a few years ago at the Walmart I used to work at, one of the self checkout cashiers had a gun pulled on them just because they walked over to the customer to see if they needed help. Thankfully no one got hurt!
It's always been scary out in the real world. You're just finally seeing it I guess. When I was a pre teen I didn't think anyone would shoot up a school. But they did.