On the one hand, if he did rob the store, that does not make later use of deadly force justified if he was not a threat to the officer, or had surrendered.
On the other hands: 1) it does suggest a motive for the kid to have been resistant to the officer. If he had just robbed a store, and was worried the officer was responding and he was about to be arrested, it might explain his effort to block the officer, and then in his panic to fight the officer; and 2) it casts doubt on the friend's initial account that the whole thing started over whether they should be walking on the sidewalk. If, in fact, the officer was responding to the call, saw the two, and pulled up to confront them, that would indicate it was not some random encounter over where to walk.