Not only is the titular barnacle a being of intense, unfathomable hideousness, it is capable of committing total omnicide, and does so by the end of the story, regardless of whether or not this was its intention.
"That didn't help at all!"
Patrick was simply reading the story like any other cheery fairy tale, and ended it with a really, reallycreepy smile. Spongebob's reaction says it all.
If a mere barnacle can kill everyone, imagine what other creatures can do...
In addition, if everyone dies, that means the Barnacle is included. So try to picture this for a minute: not only should he suffer the guilt of killing anyone, but also he died with no chance of redeeming himself. The logical conclusion is that to be killed by the Barnacle's ugliness, you must look at him. This means that Barnacle should have looked at himself in order to die, thing that doesn't happen accidentally. The most obvious conclusion is that the Barnacle killed himself out of guilt and/or loneliness.