Catching legendary pokemon in Pokemon Go is not easy. Even if you can throw an excellent curveball every throw you still have a 50% chance of it running away.
There are many stories of players who had a mewtwo or other legendary run away after 13 or more excellent curveball throws in a row.
The reason it ran is not bad luck... It's just bad timing!

*** IMPORTANT NOTE:
This app is not about timing your throw to coincide with the Pokemons attack! Do not download this app if that is what you're looking for!

This app is about decoding the algorithm that Niantic uses to implement a catch rate.

The game designers calculate your catch chance using an algorithm that despite not being publically published we can observe the behaviour of by observing the pokemon catch animation for properly thrown pokeballs.

More than 12 months of research has shown that there is a repeating cycle within the catch mechanism and that a pokeball thrown in the wrong part of the Catch Cycle is a wasted throw.

Based upon how the pokemon reacts to the throws you make PokeCatch Helper tries to calculate when to throw and when not to throw. Greatly reducing the number of wasted throws and significantly increasing your chance to catch.

PokeCatch Helper does not help you to throw Great or excellent throws. You have to develop that skill on your own and there is plenty of youtube content aimed at that. There are even links to those videos in the youtube videos for pokecatch helper.

PokeCatch Helper is not suitable for fast paced gameplay (i.e. "Raid Trains") due to it being extremely time consuming to use. Many active users simply wait until they have a High IV Pokemon to catch before utilising it.

PokeCatch Helper is an overlay that sits over your screen while you're in the post raid encounter sequence in pokemon go.

It has absolutely no interaction with Pokemon Go other than providing a button to launch Pokemon Go or switch to it if it is already running and completely relies on the player interacting with the app for it's operation.
It doesn't screen read or screen shot Pokemon Go nor require login credentials.

PokeCatch Helper and DivineEntanglement have no affiliation with Niantic Nintendo or the Pokemon Company and acknowledge the great work that these companies have done in bringing Pokemon Go to the world.

PokeCatch Helper is intended to enhance a players gameplay in Pokemon Go without interfering with the game. i.e. you can't get banned for using this.