Nooooooo! You killed that trainer at the end of the video! Now you cant get mew D: (Well, unless you pull off a pain in the ass glitch, but this one is way easier and way earlier, and lets you get a bunch of cool pokemon early too, but mew is the main one). You might want to restart offscreen and do everything again, or switch to an earlier save-state and perform the glitch.
The glitch is, you catch an abra, then save your game. Walk down , and as soon as the trainer is in sight, use abra's teleport. He will see you just as you teleport away. Now, you must go back up, and go to the route to the right. Dont battle any trainers yet. once you see a youngster looking down in the center of a circle bush-formation-thingy. Battle him. Once he is defeated, go back to the other route, and as soon as you get onto the route, a wild mew will appear. Catch it, and it is yours.
You can repeat the previous, but battle any other trainers on that route instead, for other pokemon. All the pokemon are level 17, so they are relatively easy to catch.