If you read the books in the library, you learn that the worlds weren't always empty. Sirrus and Achenar murdered every last inhabitant of 3 populated worlds. These are the only worlds that they haven't completely destroyed. All of those burned out books on the shelf used to describe the other ones.
If you get one of the bad endings, the game doesn't stop. You just remain trapped forever, until you turn the game off.
The remake of realMyst throws a quote from one of the brothers at you after the scene ends, then resets the game.
Myst is fairly bright and happy, with nice ambient music. But the sense of loneliness can get to one after a while. Then there's Achenar's rooms in the Mechanical and Channelwood Ages, which are filled with torture devices, and his room in Stoneship, which features a shelf full of poisons under two lamps made of rib cages. The music in Achenar's Cache is probably some of the scariest you'll ever hear.
The sparse music in Myst can amplify the Beautiful Void aspects to the game. What little music there is tends to be haunting, dramatic, and sometimes even suspenseful or foreboding even when you know no one's there and you're simply about to discover a strange area or machine. Other times, the only sounds you're hearing are completely natural and you are alone... and then you read the journals and know you're alone because the brothers wrecked everything in their path and it's crossed with this and Tear Jerker as you are and always will be the only soul around.
Try the bad endings of Myst with the brothers, where you return the final page to Sirrus or Achenar and get trapped forever in that brother's book. The worst part of that is that the brother laughs at your foolishness and then proceeds to rip the pages out of your prison, one by one by frigging one, while tormenting you the entire time.
The hologram of Achenar trying to speak the Tree-Dweller language in the Channelwood temple.note even, or perhaps especially, if you know that part of the recording is "Rush Limbaugh understands" in reverse. As if the music in there, the altar that eats its sacrifices, and the masks on the wall weren't creepy enough. The music, by the way, is a remix of Achenar's leitmotif.
Atrus himself is kind of horrifying in Myst, when you realize that he intentionally trapped his two sons forever in a hellish blank landscape of nothingness forever, and when you free him and return to the library all that's left of his sons are scorch marks from where he burned them alive.
The edge on the Atrus trapped his sons thing is somewhat dulled when Atrus explains that he never intended his sons to become trapped in them in the first place, and in order to even use the books Sirrus and Achenar actually had to trap Atrus and Catherine first:
It's even more dulled by the retcon that their prisons are in fact full Ages with no Myst book (which you visit in Myst IV: Revelation), and he merely destroyed the linking books in the library.
But nonetheless, where they were trapped all alone for over twenty years. The already unstable Achenar had a full-blown mental breakdown and slaughtered half of the creatures on the island before regaining his senses, several years later.
In the Stoneship Age, Achenar's room has a little hologram that starts out as a rose, to represent Sirrus. As you slide the switch on the bottom to the right, the rose slowly morphs into a skull. As if it weren't enough that Achenar had destroyed all the inhabitants of the worlds, this is pretty much a subtle way of telling his brother, "You're next."
Or it's just a creepy gadget. As one Let's Player noticed, Achenar is always the one to have the most advanced or coolest gadget in his room, compared to Sirrus: the simulator in Mechanical (compare with Sirrus' wind-up birdy thing), the hologram device in Stoneship and the hologram+speaker device in Channelwood (probably used to scare and confuse the natives). It's probably just meant to disturb the viewers, not meant as a very passive-aggressive threat to his brother.
In the Mechanical Age, Sirrus' room has a telescope that usually shows nothing. If you rotate the tower to the North, though (which is 180 degrees from where it is usually pointing, and Atrus' journal even says the South is where the tower was pointed most of the time), the telescope is now pointing in the direction where Achenar's room usually is, and a quick glance through it reveals a skeleton hanging from a ship. As if the decaying head (pictured in the page image) just wasn't enough.
Although, if you'r playing the Steam version, the fact that you get an achievement called "It's Bob!" can lessen the effect.
