EDIT: Reading over this, it came out a bit more windy than intended, but hopefully is still useful.
"Aren't we all pretending all the time?"
That's one of the great things about having reasonably anonymous message boards--there exists the very good opportunity to perform communication without the overhead associated with being polite, reasonable, or even civilized.
On Hacker News, I expect that my posts will (for better or worse) someday come back to haunt me. Indeed, if you apply to YC, I believe one of the application questions asks for your HN username. This being the case, there are certain arguments and jokes that I have had to refrain from making. I've even had to apologize OOB every so often to people here for being hot-headed (I believe I once said some nasty things to one of the senior Apache folks...awkward).
On 4chan, these "vile posts" are honest expressions of either the poster or of the facade the poster wishes to maintain--and in a completely anonymous setting, it's easier not to pose at all. An interesting research question would be to compare the normal posters to the non-anonymous "tripfags" and "namefags" (users who post under the same username and signature--terminology is as found on the chan), but I honestly have no idea how that might be done in any useful sense.
The entire mindset of interacting with complete strangers, and especially knowing that you may well never share a thread with them again, is hugely liberating. You can make and laugh at jokes that otherwise would be in poor taste. You can discuss trivial things very seriously, and serious things very lightly, and be assured that you won't be tagged/branded permanently as a fool or a fetishist--consider what we've done to losethos here, and how at this point even if he started posting coherently he'd still downvoted into oblivion.
"Are you implying that I'm only truly me when I'm vile
(for example)?"
As explained above, I imagine that the proper insinuation here is that you're only truly you (publicly) when you have no incentive to put on a ruse/pleasantries--and that if you do so anyways, that is part of the true self.
My experience, for example, is that I am just as much the humble helpful hacker given debugging help and useful links on the /pr/ programming board as I am the mean-spirited hateful troll on /adv/--and on any given day, I may do the opposite. Seeing my behavior when held to no external standard has been enlightening, and proven to be a useful data point in self-discovery and self-betterment. I try to help more and troll less now. :)