Was the opinion-based close vote reason "ridiculous" for the question you read? Probably yes.
I am the first one to have voted to close, exactly for this reason. Wait, there's a contradiction, isn't there? Welp, It's just that I voted to close for the 1st revision.
I don't know what happened to the question after my vote, but knowing it leaped from "How can you kill someone with something that isn't real?" to "Is it plausible for one's own vivid imagination kill them?" implies there were big changes meanwhile. Therefore the close-vote reason is likely to be at least obsolete, if not the closure in its entirety.
Also note that because official closure reasons are not exhaustive and because they're chosen by the majority, they are explained very often nowadays. I do it every time, and if I didn't, I upvoted the comments that matched the closest the reason I chose. For this question, it was something akin to this :
Disregarding the logical impossibility depending on how you look at it (Willk's lies is a neat example1), it looks more like you're brainstorming, something SE isn't really good at, as per the help-center says. To focus on a more specific issue, perhaps start by giving more details about what you mean by "not real" and who and from whom the crime is about?
I put in italic the words I'm not sure anymore, it's copied from my butterfly/goldfish memory 🦋.
Was and is the opinion-based close-vote reason "ridiculous"? No.
And I mean both for the 1st revision and in general. If you wish for an official SE source, the best one I think is this one : Good subjective, bad subjective. But it's 12 years old now so likely outdated to the new norms and it's not exactly the way I think, anyhow :). I do entirely agree on this very sentence though :
Most forums and chat rooms have a scale problem. As in, they don’t. The more people that join the discussion, the more noise each of those connections bring. So the forums get progressively noisier and noisier, and suddenly one day … you stop learning.
From my personal experiences in both forums and real-life as a tutor, the more you give to someone at once, the less they actually receive. There's a limit to how much someone is able to learn, if not even willing to learn in a information overloaded world 📙⚡😖.
As such, this site is a lil' bit different from others : Ask questions, get answers, no distractions. Well, this is the ideal, at least 🐶. Point is, the site is designed with that motto in mind, going from "game-like" mechanics like score to how and which questions are displayed to the readers. Opinion-based questions in which every answers are equally valid strongly lends towards distracting discussions and prevent clear categorizations. Unless you want page-long comments arguing on what wants the asker or disappointing feedback that it's not what they want "just because", it's a thing to part with, both sadly and for good measure.
In this question's case, the querent wasn't providing any context to what they wanted to achieve, they basically asked the question in title. This makes the question unnecessarily a brainstorm regardless if it was answerable or not, because it would have needed very subjective intrepretations in order to nail an answer, biases which doesn't represent the actual question behind. If I wasn't closing for opinion-based, I would have closed for either being too broad or lacking details, with a very similar advise at the end; Indeed, these two other reasons complemented opinion-based, as no clear goal was given : What's important to them when they want to kill someone that way? Is it efficiency, mass-murder, speed, sneakiness? Something else? What should be prioritized? And what is already existing, in order to determine by the opposite what can be answered?
But most importantly -and that's where the objective part leave the stage for my viewpoint-, since they haven't showcased a choice made on the starting point and a direction, how can one reliably answer and not hinder their creativity ? Not giving anything highly suggests they're still in the stage of choosing than inducing, ie. brainstorm.
Issue is I don't know them, nor do I know their project -if they have one-, and so the choices that will attract them the most. Supposing that Willk's example is the correct interpretation, that the crime tool just needs to not be "material" or "physical", giving the possibilities I could think of would inherently restrict the choice they would make. Yes, even if am -or was 🦋?- able to reverse the chessboard most of the time. I would limit not because of rational thinking based from facts about the world, but because I will necessarily impose my viewpoint on what they should do from what I, the "expert at worldbuilding" would do. Problem is, I'm not an expert of their world.
I'm not here to restrict the end choice they are free to make, and I think anyone else shouldn't, either. It's a very quick-sand way of drowning one's motivation, their inner goal not being the same as the world they're creating. It's the most important thing, ever2. In other words, it's the most counter-productive way of helping someone build their world.
This is why I intimately think opinion-based is a valid close-reason. Are the official statement unclear or wrong? Yes, I do agree it's very misleading, and the help-center telling every answers are equally valid is a much better explanation. Does it mean it's unfit in its entirety? No for the above reasons.
It's annoying, but we have to compensate its usage with a comment explaining why. But remember it's like any other closure reason, wherin we need to give which timeframe the reason is valid in case of major changes, and just what exactly is the issue and what should be done to improve the question 🐨.
1 : Willk's example was a link to lies spread for the Covid epidemy, which in turn led to deaths. For them, it was something "unreal", but I contradicted saying that the fact in the lies don't exist, but the lies did, and it's the latter which killed people.
2 : For a complementing reference, the very last and most important objective of "The Art of Game Design" book by Jesse Schell ends with this : "Why do you do this?". My book is an old edition, but I doubt it has changed in the newer ones.