Basketball players shouldn't be defined by their worst moments. No one should be. Everyone is more than the sum total of their finest moments or biggest failures.
When Andre Drummond airballed a couple free throws, I argued that while funny, Dre's poor free throw shooting doesn't undermine his overall effectiveness on the court.
Similarly, it wouldn't be fair to Brandon Knight to write him off because he airballed a wide-open, breakaway layup. Or because Kyrie Irving literally made him fall over while trying to defend him. Or because DeAndre Jordan did what he did in Lob City. Or, well, all this stuff too.
That said...
I would argue that the video above, while not the last word on Brandon Knight as an NBA player, is a pretty good representation of what we've seen from him so far.
One minute, he's displaying all of the talent and potential that made him the lottery pick he was. In this case, we see that he's capable of being alert on defense, he's lightning quick, and in a foot race down the court, he's awfully hard to catch.
But in the next minute, the wheels fall off, and everything is terrible.