Any untrue/invalid idea is bad. So it is not always subjective. “Tigers are friendly pets and every apartment in SF should have one” is an obviously bad idea.
But for who? That makes it inherently subjective. For your obvious bad idea, there are a bunch of contexts where that is a good idea, even though it might be hard to imagine from your own perspective.
Your example is bad for people who live in SF (most likely), but imagine there was a rival town that really hates SF, and wants them to be eaten by tigers. Then the idea of having one tiger per apartment makes a lot of sense, it would almost certainly lead to lots of people being eaten by tigers.
But this is silly. It's obviously a bad idea.
Here's another obviously bad idea: "vaccines are bad and cause autism". You could argue it was a good idea for the fraudster who started this notion, because it gained him fame, but nobody thinks in general about ideas like this. It's obviously false and detrimental to mankind, and therefore a bad idea.
The Heaven's Gate cult also had an obviously bad idea. Mass suicide didn't teleport them to the mothership. They killed themselves for nothing. Had they lived, maybe some of them could have been rehabilitated and found happiness in things that weren't falsehoods. Or not.
But that's a fruitless line of thought in this context.
It doesn't negate the fact there are bad ideas. Heaven's Gate mass suicide was a bad idea (and its justification was patently wrong: no soul was uploaded into the mothership). Bringing feral tigers into your apartment is another bad idea. They are only "subjective" in the most theoretical, I'm-having-an-internet-argument sense; in practice they are objectively bad ideas.
Some ideas -- maybe most -- are neither good nor bad. But again, this doesn't negate that some ideas are objectively bad or good.
Hardcore moral relativism (or is it "ideas relativism"?) is such a boring position to hold. I suspect it's also a dishonest position, in that deep down very few people actually hold it.
From your POV and mine yes.
From their POV no. Clearly that's why they did it.
I agree what they did was probably a bad calculation and incorrect interpretation of the world given what I and you know about the world.
So from our POV it was a bad idea.
And from their POV it was a good idea and that's why they did it.
If they can reflect on it now they would likely call it a bad again.
Again subjective based on what they know about the world now.
No. You're confusing what someone mistakenly believes with the quality of the idea.
In this case, the POV doesn't change that this was a bad idea. Someone can think at the time something was a good idea, but this doesn't make it so, especially if it was demonstrably a bad idea. Few cases are clearcut, but this was.
The Heaven's Gate cultists thought their souls would get uploaded, that's why they killed themselves. Their souls weren't uploaded; therefore by their own logic committing suicide turned out to be a bad idea. They just aren't here anymore to reconsider, which demonstrates it was worse than a bad idea: it was a terminally bad idea.
Another example that POV doesn't necessarily change the quality of an idea: a toddler finding and handling their parent's loaded gun is objectively a bad idea (as is the parents leaving loaded guns within reach of toddlers, if they did it on purpose out of carelessness). The toddler may not know, from their POV, that this is dangerous; but this doesn't make it a good idea.
Some ideas can change with the POV, but you're mistaken in believing every idea does.
Moral orientations fall neatly on two spectra. One is empathy to narcissism. The other is practicality, which can be counterintuitive, vs emotion, which has no concept of consequences beyond the present moment.
Both are "subjective" in the sense that you can't usually argue people from one to the other.
The rest is execution and PR.
There's some social conditioning which teaches people that empathy is good, and rather more that suggests practicality is valued and emotional regulation should be a thing.
But in the real world - and especially in the US - narcissism is far more valued in practice, and emotionality is far more profitable.
Which is why it's a culture of opportunists, corporate criminals, scammers, and manipulators.
Meanwhile, in a very real, literal, sense Wall St is just Heaven's Gate with quarterly reports. The outcome - mass suicide - shows every sign of being identical in the medium term.
The machinery is far more abstracted and indirect, which makes it harder to see when you're inside it. But there's only a difference of scale between a lone nutjob scamming a handful of followers to death and a collection of systems and institutions that are objectively likely to leave most of the planet uninhabitable by the end of the century.
I don't think I can buy into this argument. If nothing else, Heaven's Gate did NOT inoculate other people from joining cults.
But it's a patently bad idea anyway, "this can kill people but it's good because it will teach other people not to die".
If at some point humans had articulated this into "an idea", they would have seen it as a good idea. Which it would have been, since it's unrelated to wolves attacking humans. It's not that humans said "hey, let's go pet a wolf pup, I'm sure their parents won't mind!".
Little white lies are objectively okay.
They are subjectively ok. Some people believe lying is always bad.
I think the "true idea" (or let's call it "reasonable idea") when fully spelled out is "sometimes a white lie will be beneficial, will cause less harm, will inspire confidence in the receiver; and conversely, the harsh truth may harm them or be counterproductive". It's not the white lie itself that is the "good idea that is untrue"; the good idea is the reasoning behind the white lie, which is true.
Simply because someone believes something does not move the phenomenon to the subjective realm.
The Earth is not flat.
Some may argue a case for why they believe lying is always bad and would never engage in it. Yet experience proves those most vociferously against little white lies are often lying to themselves on a variety of topics. "Honey, does this make me look fat" is a no-win situation for these individuals.
From your POV yes. From their POV no.
I'm sorry, you cannot use this argument. It's not even in the same category.
Whether telling a white lie is ok is as subjective as it gets. You can pick better examples if you want to make the case that some ideas are objectively good or bad; this one is not it.
What good is that when you have to live with a tiger?
Capitalism is neck-deep in bad ideas: “buy THIS product, it will add years to your life expectancy!”
To the people selling the product it’s a good idea even if it’s a load of garbage—a quick buck to be made; to the person buying it, terrible idea in fact. :)