Accessing the camera on the Galaxy S6 and S6 Edge is possible by quickly pressing the home button two times. The camera app will open upon the second press, even if the screen of the device is off.
But this also leads to mistakenly launching the camera app when you're trying to unlock the device, or readjusting your thumb in order to get the fingerprint sensor to work. I've done it at least a dozen times.
Thankfully Samsung has included a way to disable the camera quick launch feature.
There are two methods to turn it off. The first, and quickest, is to open the Camera app, tap on the Settings icon, and slide the Quick Launch switch to the off position.
The second method requires you to launch the Settings app, select Applications under the Device category, followed by Camera. Move the Quick Lock switch to the off position and close the app.
With quick launch disabled, double-pressing the home button will only result in your device going back to the home screen. Of course if you want to enable the feature, follow these same steps, with the end result being the same switch moved to the on position.