YOU MUST HAVE STEAM CLOUD ACTIVATED FOR THIS!!!

I wanted to complete the game without having to run through it twice so worked out how to back up and restore the game files, hope this helps someone as it too me a while to work out.

Make sure you choose Green when you have the option of either Green or the Murderer at Sara's place otherwise you will be locked out of the good ending.

When you get to central park and the auto save kicks in - exit the game and let steam cloud sync.
Once synced then go to https://sp.zhabite.com/steamstore/account/remotestorage and download your save file for the game, I downloaded twice so I didn't mess up.

Once you have done this, then you can progress as normal and either choose to shoot or not to shoot. After the credits have rolled and you have listened to the voicemail then exit the game again.

You will need to recover your save now using the backup, for me the location was
C:\Users\{your user name}\AppData\Local\detnoir\Saved\SaveGames

Just paste in the downloaded copy of your save here and overwrite the existing, this will revert your save back to where it was at central park and you can choose the alternative ending (shoot or throw away gun)

Hope this helps some people and saves them a load of hours for one achievement like it did for me.