Yes, it
is ok.
If the only thing stopping me from doing horrible things was that I haven't yet imagined doing them, then I and everyone around me should be terrified of me. Fortunately, my self-control seems to be a little stronger than that. Though it is not helpful for everyone to be told by implication that they have permanently terrible self-control (or any similar implication). [1] Fortunately I seem pretty good at dealing with such implications, although I am less confident about other people.
(Some articles have repeatedly appeared on HN, to the effect that when gifted kids are taught that "gifted" is an unchangeable property of a person and has nothing to do with gaining skill by repeatedly playing with something, they tend to stop putting a serious effort into doing anything new and challenging--for fear they'll screw up and have to accept they're not "gifted"--and so they tend to stop or severely restrict their mental growth, becoming rather less "gifted". Now imagine how this applies to self-control.)
I just did picture murdering my family over dinner with a fork. I decided the idea holds no attraction to me, and is pretty stupid. I am not worried about deciding to do it in the foreseeable future. Feel free to explain how this situation is not ok.
(Nor do I think the idea will repeatedly come back to me in the future and I'll panic and try to think about anything but it, and only end up thinking more about it, until I lose sight of anything else and just do it because I don't know what else to try. --I have the impression, from some books I've read, that that's one way some initially sane people end up doing crazy things. In this scenario, the panic is obviously an exacerbating factor, and it stems from the belief that "I'm thinking about something horrible [or have been thinking about it for what seems a long time]" => "There's something wrong with me and I won't be able to stop".
My solution is contempt for that belief, and for other people who believe it. Like, possibly, you. (This should be combined with some sort of escape valve--if you conclude that everyone else sucks, you need some way to deal with that, and if you don't have such a way, then you may start panicking if you come close to thinking that everyone else sucks. I just assume that, no matter how high a percentage of other people appear to suck, there are people like me, and eventually we'll get together and make a good society). Meanwhile I have contempt for the horrible idea, because it is not special amongst the many other horrible ideas--let's imagine them!--and if I were vulnerable to doing them, then I'd probably have done one of them already, which I haven't. And in the unlikely event that I do keep thinking of it a lot, my working hypothesis will be that it's just self-caused by anxiety foisted on me by taking seriously, on some level, the words of people like you. ...I have read of a case where someone said that he did keep having murderous thoughts, and that this coincided with the development of a tumor in his brain; I guess it might be possible that the latter could cause the former somehow, though now that I know of this, I would still not panic, or if I did it'd be about cancer, not about the murderous thoughts. A brain tumor with such specialized effects seems very unlikely in any case.)
Anyway. This little exercise, picturing your scenario, reinforced my confidence in my sanity and my ability to decide not to do other horrible things. I'd say the experience was positive for me. So, why? There you have one answer: an exercise to prove one's sanity. (Again, I do not respect someone who would do horrible things and just doesn't because he hasn't thought of them yet. I would hope such a person would learn and grow stronger than that.)
And there are use cases for putting detailed, horrible, evil things into media productions--not breaking reader's immersion, teaching people about their vulnerabilities so they can protect themselves, and sheer enjoyment for other reasons I'm not sure of. I wrote up a couple of examples, but they made the comment too long, so here's just one, which I write about because it's fun:
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In this clip from the anime series Death Note, the main character Light Yagami, who (unbeknownst to anyone else) is the evil killer Kira, confronts Naomi Misora, a former CIA agent and the fiancee of an FBI agent whom Kira recently killed. She has some very compromising information about Kira that she intends to bring to the police; Light wants to kill her to prevent this, and he needs to learn her real name to kill her.
https://www.youtube.com/watch?v=q1ISK8Z0gN4&t=8s
Light lies to her, learns some personal details about her, compliments her, learns more details, learns that she trusts him (for the wrong reasons); exploits her trust and what she's told him to make her a (completely fake) offer he knows she'll really want; pretends to back off from it to avoid suspicion (while continuing to compliment her and extend the offer); and adds a comment ("But you're young and beautiful... don't risk your life for this") that sounds like more backpedaling, but which he knows will actually bring out her fierce loyalty to her fiance's memory and induce her to say yes. She accepts, and tells him her real name; he writes it in his Death Note (pretending it's a normal notebook) and kills her, making her jump in a river so it looks like suicide.
I consider this pure evil--in particular, noticing her admirable qualities of trust and loyalty and determination, and making them work against her; and conducting the whole conversation with a straight face until he kills her. I also find it kind of beautiful. Meanwhile, I would be repulsed at the thought of me or someone else actually doing this sort of thing (I'll classify it as "observing and manipulating someone through conversation to get them to give you what you need, then seriously harming them") to someone I knew.
What is the value of my seeing things like this? Well, I don't expect to understand fundamental reasons why I like things, but I can make up guesses, and an obvious one here is: Knowing an archetype of pure evil, I can be better prepared to deal in real life with examples of impure evil that approach the archetype to varying degrees. (Also the pattern-matching part of my brain will be more likely to notice if I start doing things that approach evil. Also, it might help me understand other people's reactions if I unintentionally do things that look like evil. Though, for the record, looking like evil is not evil, and using force to stop or punish someone who merely looks like evil is unjustified.)
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I'm glad Falkvinge is addressing this issue. It had to be done, sooner or later. And he makes a powerful case--especially section 2, I dunno about section 1. (I'm impressed... I know of the "humanitarian with the guillotine" pattern, where a naive, doesn't-examine-the-secondary-consequences-before-acting, but generally goodhearted politician makes a well-intentioned law that ends up hurting people more than it helps. I'm familiar with the mental tool of imagining that a law was made maliciously, to figure out what the bad consequences might be. But I keep being faced with the conclusion that it's more than a mental exercise: that all pieces of legislation were in fact originally put forth by people with anti-social intentions.)
[1] John Holt explains, with several examples, how some groups of people (but not others!) have come to think that children are reckless and uncoordinated and dangerous to themselves and others, and how their resulting treatment of children causes children to exhibit exactly those traits, in ways they do not when they are not so treated. http://pastebin.com/LkBd4VhN