At first I was confused, because the giants literally created a path for him, and I was wondering if he was just showing off or what.
After a bit I understood that the Giants were pretty much guaranteed a touchdown at that point, and that strategically they were better off running down the clock as much as they could before getting it so the patriots wouldn't have as much time to counter it. Since this touchdown was a given, the patriots were better off letting them have it as soon as possible so they got as much time as possible to make a touchdown themselves.
He was intending to knee the ball just under a yard from the goal so they could continue to run the time down, but since he ran unopposed and misjudged his speed, he couldn't stop in time. You can see he's not just reversing, but getting down to drop his left knee. He just had too much momentum.
If the Patriots had won, everyone would have considered this moment to be the breaking point of the Giant's game. That's why people were upset with him - the play was running down the clock, not getting a goal, and he didn't make the play.