Ok, this is an extremely obvious topic yet it does not have a thread. I think this is one if not THE single most important mystery in the series. Lets consider the circumstances of the ryodan to start.
1.- It is made of EXTREMELY powerful nen users. The least powerful of the nen users there is killua's little sister but considering hisoka considered her "tasty" I would think that she is close in talent to killua in terms of talent. Killua and gon were considered by nobunaga due to talent too. They also value rare abilities and it seems they make decisions based on that too. For example, shalnark mentioned how his ability and that of someone else (don't blame me for not remembering) would be easy to replace thus their loses would not be that great for the ryodan while shizuku's and pakunoda's abilities would represent a huge loss for them. In the end, not a single one of them is a weak fighter.
2.- They do a bit of every activity. It is not that strange that they rob and kill to their convenience but it is very weird (IMO) that they also do charity work (stated early in the manga).
This along with the previous point is important to determine one thing. The Ryodan is not just a band of psychos who kill for the heck of it. That could be one reason to kill but ultimately they act for the greater whole of the group and their objective. Also, the link shows that the ryodan, its goal, is in itself more important than any member's life, even the leader. Even if dancho were to die the ryodan would continue to function normally towards its goal.
Now, based on this part it is clear that the goal is not the random ambition of the dancho. If it was he would not make a point out of his life not being that important. They have a goal, but not just any ambition. They have an ambition which the group as a whole believes to have inherent objective value. It is something which they value above their lives. Isn't it weird that this group would be so eager to give their lives for their team mates? They are a bunch of killers yet they seem to genuinely care for each other, the relationship between members is very brotherlike in many ways.
Now, as for my personal believe I think that at least they have an ideal of sorts which they want to uphold. They want to build something which they believe to be transcendental. This might be too political though.... It can't be just the random destruction of the world since that would destroy the spider too right? I am thinking something along the lines of a revolution here. Ryiseigai is a place where things which are disposed of end up. No one knows of the circumstances there so the ryodan could be doing anything there. They could actually have organized the place into a nation and no one would ever suspect. Of course something that huge could never really be done or funded with the work the ryodan has been shown to do, there simply is not that much money in that. What does everyone think?
It never actually crossed my mind that the kid with a kimono was a girl lol. I guess that even if he looks like a girl, he moves like a girl, he fights like a girl and even dresses like a girl the person in question is not necessarily a girl lol. Anyways, that is largely irrelevant to the actual topic of the thread...
There's a flashback showing the Spiders as kids, saying they only wanted... (we have no idea what)
Beyond that there's nothing much about why they exist. As an aside, I'm not sure what's so extremely powerful about a group that will lose 6 guys against a new Hunter as prophcised by Neon. That said, the whole thing Kuroro talks about the whole group must survive is basically completely wrong because there are only two people in the entire organization that can lead: Kuroro and Franklin. The moment Kuroro gets captured you see the Spiders basically split into two groups and start trying to kill each other, and only Franklin's wisdom prevented this worst case scenario. In fact, it seems likely the 'lose 6 members' really refers to the fight amongst the Spiders themselves, as I don't really see Kurapika being able to take out that many members by himself.
So while Kuroro has a great vision, it's meaningless because he has no successor and the organization will fall if he falls too.
I don't think the Spiders have any objective--except to steal, kill and wreck havoc.
Didn't Gon ask Chrolo why he killed and Chrollo thought to himself how he never actually thought of a reason.
But that would just not make sense. Such a thing would not explain how the spider works nor the feeling of camaraderie between members (when they are not fighting). Why would Uvo die for the spider if the objective is merely to keep stealing and killing for no reason whatsoever? I guess they could be something like a gang but with all loyal members but I just can't imagine them not having a real objective which would justify the idea that the ryodan is more important than any single member (even the leader).
Uvo wasn't going to sell out his comrades.
They don't need a goal to not want to sell each outher out.
As for the group mentality of the Spiders. It's a counter against hostage takers.
A hostage does no good to the hostage taker if they have no worth.
If it wasn't for Pakunoda, Chrollo may have been killed by Kurapica but Gon and
Killua would be killed by the Spiders and the Spiders would then likely kill
Kurapica (I don't think he can beat all of them at once.) Now Kurapica might
do that because he's bent on revenge but a lot of other people wouldn't go that far.
Not everyone in the Spiders feel exactly the same toward comrades. I believe the original members of the Spiders have very strong sense of comradeship, but the new guys Kuroro added are not nearly as loyal. During the split after Kuroro was captured, the guys who wanted to trade hostage were Paku, Machi, Nobuanga, Franklin, and Kuratopi. That's 4 out of the 7 original members there based on the flashback (and 2 aren't phsyically present). I'm not sure if the 7th original member was Shalunark (hard to tell from the picture), but you can see there's a considerable difference in the attitude between one of the original (preserve the whole at all costs) compared to the 'kill them all' guys.
It's nearly certain if the hostage exchange didn't work, then the events would have unfolded like the prophecy. Pakunoda sacrificed herself to prevent 4 other members from dying. I think she chose to betray what the Spiders stand for so those 4 other guys didn't have to die. To me, the 'glory' path would be to not negotiate and just fight Kurapika, and it's stated mutiple times that fighting Kurapika head on would've resulted in 6 total deaths (Ubo + Paku + 4 others).
I always read it with the impression that they were trying to help the people on their original country, actually. Don't ask me why, it was just on my mind since I read the name for the first time.
Not sure how, why exactly and what they're doing but I always assume it was for the sake of the country of the stars.