He is an optional first boss; I.E., you don't need any specific items to get to him, much like the Tauros Demon. While it's true that rookie players may not be able to get to him right away, he is still optional nonetheless. Quelaag IS NOT an optional first boss. Why? Because she requires an item in order to become your first boss.
Actually, you can head-shot ingward with hunter starting equipment, open the flood gates, and access any of the following bosses 1st without master key:
Quelag
Sif
Moonlight Butterfly
Taurus
Pinwheel
Now, while blowing by the ghosts in new londo and sniping ingward isn't exactly plausible for a beginner, neither is doing a runby through hordes of skeletons, making leaps of faith into short cuts, or running the catacombs w/o shortcuts with pure starting equipment. That you need an item is a rather arbitrary distinction. The first boss you fight is your first boss, and any of the above are "options", though for a beginner only the burg is really plausible.
Everyone says Pinwheel is easy because they usually fight him after Anor Londo, when in reality he's supposed to be fought after the Moonlight Butterfly.
That's one take. However, your immediate priority according to what the game tells you is to ring the two bells of awakening. This implies gargs and quelag, and the path to reach them. One could easily wander down into Valley of Drakes from the basin and go right to quelag. The crestfallen merchant implies its below the burg or some such, so the reasoning most people would use is to enter the depths through lower burg, which starts that path line. Regardless, at that point pinwheel isn't a first boss anymore, and the catacombs are still significantly harder than the depths regardless of whether you shortcut either.
Witch of Izalith was probably sane right up until she got consumed by chaos, and not after. Nito probably doesn't fit the definition of sanity, as we don't even know that he's a thinking being for certain or any of his motivations, if he has any and isn't just a force unto himself. Seath is definitely insane (even the game says so I think), and 4 kings...well, might or might not be depending on what kind of lore you believe. The might just be purely evil, but not necessarily insane.