How can I earn kills with Hanzo's Scatter Arrow?

Shoot the ground directly in front of the opponent. Since its a scatter effect it will work best against big targets. It's a little tricky to do, as the idea is that the arrow will impact the ground so close to the opponent that all the scattered arrows don't have a chance to go into anywhere else. I would recommend loading up the training grounds and trying it on the training dummies to get a feel for how it works.

Edit: Here's a crude drawing of what you're trying to do. The red line is the scatter arrow. It's like you're firing a shotgun shell at the point of impact so the closer the impact point, the more effective it is.

Yeah I had the same issue, but thanks to a quick start guide I noticed this same mistake.

They key is to aim the arrow in front of your enemy and to the ground. That way most of the scattered projectiles will go to the enemy you want to shoot.

What you've been doing so far is to shoot him with one arrow(scatter), and every other arrow goes everywhere else but him, because it scatters from him towards every other way.

I tend to combine the "shoot the ground in front of them" technique with "try to shoot the arrow in small rooms" For instance if someone is running from me in a small room I just randomly shoot in the room and let the scatter do the rest.

Check this youtube video out.

In addition to the other two answers, I'd also like to remark that you should really look to try to use this ability in tight corridors, such as the side corridors on the indoor control point of Liyang tower. Even if your geometry skills aren't great, the corridor is so tight that the arrows will bounce around enough to hit whoever's there.

Also, really look for different surfaces to bounce your scatter arrow off of. For example, say you have a Reinhardt with his shield up, but he's in front of a wall. If you have a good vantage point, it'd be prudent to shoot your scatter arrow at the wall behind him so it'll bounce onto him. Just my 2 cents.