The ending.

Im not sure about Fu either, come to think of it: she certainly could use some physical protection if more assassins show up, but its not the same.
Mugen definitely needed the others to maintain/further his attempts at civility; but (after my last view-through) I have to wonder if Jin wasnt really the one closest to true 'self-destruction' in the series.

If you remember their first 'real' split - after Mugen starts arguing with Fu about the whole Sunflower Dude issue, and the two of them walk away in a huff - Jin tries to stop the other two and is left standing by himself with something of an uncertain (maybe even sad) look on his face as he watches them go.
Not long later, Jin ends up straight up jumping off a damn waterfall - and Im REALLY not convinced it was about not wanting to kill the guy who ambushed him, and more about the fact that his two traveling companions (who would eventually become his 'first friends') just ditched him and he basically has nothing else to live for.
Mugen didnt even seem interested in killing him, which is probably a double downer considering their weird 'Ill get you eventually' relationship. The poor guy hasnt really had anyone his own age that was interested in being his friends, due to his sheer badass-itude, and this was likely his first (rather shakey) step toward reaching out to someone.
That said, I dont think he's really out of his shell enough (by the end) to be considered 'fully functional' - just as Mugen needs Jin to calm his aggression, so Jin needs Mugen to just generally be his friend (something Mugen can do relatively easily, as hes not really 'afraid' of Jin like most people evidently are). :)

Indeed, Vicious lets his emotions take him to a bad place, which tends to be what happens when a strong good emotion turns into a strong bad emotion... and doubly so for drug addicts, which I would suspect that Vicious is. Why? Well, look at the bags under his eyes, and look at his behavioral decline as compared to that of Aasimov in the very first episode - id be willing to bet the boys using bloody eye, hence why hes still a danger in a swords vs guns scenario. Iirc, theres even a scene in Julia's apartment with a vial of bloody eye on the sill - would certainly explain why J's so willing to get out of a relationship with him and hop in with Spike.
Still, there is quite a line between getting bitter and pointing a gun at someone - and Vicious sprints across that line with sword in hand. :P
Thinking back now, the bird is kind of a weird little symbol of that dark nature of his - initially, he seems to actually care about it/enjoy its company. But, when the time comes, he kills it - and what small part of his humanity remains - to further his own ends; not unlike what he was prepared to do to Spike.
(I get shivers now, seeing the shot in the credits of Vicious at the top of the stairs - catching Spike and Julia together, no doubt - as thats where this whole story really begins)

Ill def be keeping an eye out for The Wind - that pic alone would make it worth it, and I can only imagine what other gems are littered throughout! Again, thanks for the posting (and for correcting me on the source of that awesome pic! ;D).