Lots of great points about how alignment should not be the guide to /all/ RP of any PC. But this is a game, and everyone has different opinins about how to play it and how to interpret the 'rules', and everyone has a lot of freedom to also make a lot of choices. In this game, alignment is still part of the "rules", and for me if you one day play a Paladin, and die, you can roll up a new PC say a cold blooded assassin the next day. Two days... and you, the same person IRL, go from a Lawful Good to a Chaotic Evil PC. We're allowed to do that no problem. I suppose the player thinks many different ideas for PCs are interesting and fun to play. And in this game, with rules, it is kind of 'cool'/'fun' to "portray" a certain PC-class for yourself and for the fun (approval?) of all the other players playing their PCs in the game world... so... we play on.
but, are we... 'supposed' to accept and act accordingly to even an alignement restriction as strict as a Paladin's? Most say "No based on IC situations."... but others would say: "You picked Paladin, you must adhere no matter what."
These alignment-hardwired (supposedly? say the rules?), like Paladin (and the old 'must-be-TN' Druid), have set rules in our game system that say you /must/ adhere to an alignment ideal (like how druids now just must have "neutral" in their alignment somewhere), or, the mentioned specific extreme, like how Paladins must remain LG or lose their powers. So, to be a Paladin in D&D, wether you think alignment is supposed to be your PC guide or not, you as a player if you want to play a Paladin that keeps her powers... then you must use LG as your RP guide. Now, all kinds of moral decisions will appear in game - and if you say to yourself: "This is what my LG Paladin would (should?) do. I don't agree IRL or want to do that, so I won't." --- is that an
INFRACTION of the "game rules"... ?

Most would say: "No, "I" can do what i want, it's my PC." ....Or, is every PC just NOT supposed to be hardwired into an alignment and can move up and down the scale on a whim, since some believe alignment is not the "rule" but a background stat that needs not be adhered to "not seriously"....? Know what I mean? All depends who you are. We are, apparently, allowed to roll a PC and plan the future (sort of). Example: "I want to make a Paladin that does fall from grace and eventualyl ask for approval for a Blackguard." Well that poor Paladin, from the start is a 'doomed-Paladin'... the player knows and may or may not tell anyone. So, how does that affect the other players and PCs around? What a shame! Everyone that Paladin interacts with, "expects" that PC to act a certain way... so puts 'faith' in a 'certain' strain of behaviors, possibly placing their lives in that PC's hands more than once, and poof, one day, that players decideds 'now's a good time', and holding the last rope of the suspension bridge as the rest of the party dashes across, he makes one final anti-prayer to Tyr telling him to get bent, and drops the rope killing 4 other PCs and a rare purple turtle at the bottom of the chasm - PC turns evil - more RP happens over a long while - PC becomes a Blackguard. Sweet. Great PC development! Your are not required (at all apparently) to adhere to your strict PC alignment restriction, because we have freedom to play any class or race or alignment and you 'just wanted' to decide when you go from the expected LG to CE in one drop of the rope. (poor turtle).
If you CHOOSE to play a Paladin... but you are in fact at heart in real life a Neutral Evil person, or hells, even you just might be having a bad day in real life... but you made this Paladin. He's LG. He's "supposed" to act LG. Do LG things. In the game world. So, can you just say: "I don't use RP as a guide for my PC actions.", and just go ahead and do anything? Or 'should' even you, the owner of that LG Paladin, RP with respect to the alignment a Paladin must have?
Some might say for "some" classes they "should"....or god forbid, ..."have to"?
hmmmm... I know a lot people here are sitting back saying to themselves: "Nobody gonna tell me how to RP my PC.".... and this is why when a DM is watching, if something is done against a PC's /actual alignment/, that PC might get a small to large "alignment adjustment". But if someone thinks Paladin is a "cool class to play, why not this time?", and rolls a Paladin... I think for especially that PC, who's powers are governed specifically due to respecting the LG ideal, that that player IS required to RP/act in game towards that specific extreme end of ultimate good. Again there is a lot of lee-way here since the game we play is so open for so much variation in everything that is possible, and with so many different and unique new personalities walking around and bumping into each other.
We want freedom to develop our PCs of course, and one of the big ones, everyone loves to see and especially play, and that opens waves of awesome options for further exciting RP is that "fall from grace" idea. Done a few times already i'm sure. It's also fully acceptable, since we /are/ in control of our PCs and are not restricted to doing such a thing.
I personally hope that if a player rolls any PC, that he/she plays THAT PC. If it's a Paladin for a few levels, then /be/ that Paladin, and go the Blackguard route only if the situations presented in game make sense for that PC to actually move out of LG, ...i just don't like a LG Paladin dropping the rope so to speak to fullfill some predetermined path to become a blackguard without real good and hopefully long term RP that "leads" that way.