I don't know what that reporter or that store clerk are takinbout. It appears the victim lunged at him when he saw him pulling that knife out that sleeve, which was after he was already shelved...and I don't think that was a cellphone in the suspects right hand. If he really said "what are you gonna do, stab me", then it looks like he said it at some point before they started wrestling. It's very unlikely he said that afterwards. That wouldn't make much sense. Anyway, it doesn't look like the suspect has a good case. Those words from the victim proves the knife was already a threat before it really got physical.
Edit: When you look at the right half of the thumbnail, it looks like the victim may have said, "what are you gonna do, stab me", after they were outside after all. But it looks like they were no longer wrestling. So he may have stepped away after he couldn't get the knife from him, then asked that. Looking at this thumbnail makes it appear the suspect may have took him on his word, and stabbed him because he dared him to, when he didn't really have to, which makes him look even more guilty. The only way you'll know is if they show the actual stabbing.
Yes, I agree the brother shouldn't have escalated this, but I think it was something that suspect said, more than the mere fact that he didn't say "Thank you". You'd have to be an a**hole not to thank a brother for holding the door for you in the first place, especially when he's from a difference race, and your race been oppressing his for 400 years. But in this country, too many White men still believe a brother isn't worthy of any respect, even when he's showing kindness, that he doesn't even deserve.