How to get the “Personal Stereo”

No hiding spaces? No bullets? No problem. Use a flare. All the nearby monsters will panic and you can just walk right past. Here's a video demonstration.

It seems the easiest way to get this is in fact to torture Hank (the guy in the gun shop) with the flashlight (torch). After about 40 "ticks" of this I managed to get the red ending (though my mental health was already quite low as noted).

The most efficient way to torture Hank is to stand to his left in the gap where there is no X prompt. Then repeatedly press "flashlight" and "talk" (X and [] on playstation), every time you switch on the flash light you'll torture Hank and cause your mental health to suffer. By mashing "talk" this way you'll be able to torture him once every couple seconds. If you're out of batteries, remember you can take a green pill and sleep to get more. I only needed a single battery (and didn't run out) while torturing hank 40 times.

Note that despite the "requirements" lists on the wiki, only low mental health is required; those are just all ways to lower mental health. It seems like it's nearly impossible to get mental health low enough without grinding some bad mental health, so I recommend torturing poor Hank to get the ending.