and now, perhaps, we know why the video was edited like it was. In the 'darkened' video, we can't see some important evidence.
Here is unedited raw video.
http://www.univision.com/content/videoplayer.jhtml?cid=2433072
What I see: At the beginning of the video, we see several kids making a run for an area the US side of fence. One of them appears to come from the US side, through the fence. I count 5 kids in total, though they don't clump enough to really get a good look. A border agent runs up at the same time. Most kids scatter, and the border agent detains one.
from about 1:07 to about 1:15, the camera shakes a bit off the border guard, and you can see one youth approaching from under the bridge. The border agent is pointing at him and yelling something. at 1:21, you can see another youth also approaching.
At about 1:21, the border agent takes a defensive posture and makes two shots, from the US side of the border. The two visible youths scatter. The border agent begins securing his captive. At 1:36, the border agent takes another shot, and the camera swings wide enough to see a youth falling down, and another scattering.
Within minutes, more US border agents appear, holding guns. Then Mexican law inforcement is on the scene, with their own weapons. By 8:30, the US agents have retreated, and the Mexican authorities are near the dead boy.
My interpretation: The border agent caught some kids either fooling around or actually attempting to penetrate the border. At the beginning of the video it looks like one of the kids comes from the US side, and through the fence. The kid who was killed has evidently been
involved in smuggling in the past. The agent catches one, and is menaced by the others. Though the video is too bad to see if rocks are thrown, witnesses on the video can be heard talking about it. He takes two shots, but the kids regroup and re-approach, but this time, using the bridge columns as cover (as this is where the other kid died). Then he takes a third shot, killing a kid and evidently ending the confrontation.
At no point in the video does a US agent ever enter the river bed. The shot is taken from the US side of the river. If a bullet casing was found next to the body, someone planted it there. At no point in the video did we see Mexican authorities cross the river bed, either, so I cannot say where the bullet casing came from, if those reports are even accurate.
While it sucks this kid had to be shot, from what I see, this lone border agent was being approached by aggressive attacking youths. As they were actively menacing and not just mouthing off, I can see where he would have been in fear of being overwhelmed had he not taken action. I can't really fault him based on this video. We can't see how many kids were approaching him, but from the video it seems clear to me that they weren't just taunting him but were approaching him aggressively.