If you ever worked in the service industry, you already know what it is. People go into stores like this acting a damn fool, to get a few dollars knocked off of their bill, or a store credit or some shit. It's probably been going on since the beginning of time, but it's gotten way worse since about '08, in part due to #therecession and and in part due to technology.
A cottage industry has emerged of websites teaching people how to scam businesses out of $$$ in the guise of "customer service": demand a discount you aren't entitled to and raise your voice until they give it to you, lest the other customers get upset; present an expired coupon and threaten to try to get the guy fired if he doesn't accept it; call a 1-800 number with BS claims about your shopping experience for a free gift card, so on and so forth.
There's no way T-Mobile really did double charge this guy and then refuse to just remove the second charge. And it looks like he physically assaulted the T-Moblile employee, before the guy stabbed him. He didn't count on the guy being a "freshout." That picture they showed of him looks like a mugshot.