prosfilaes
Adventurer
Second, it is still insulting to someone to tell them that their conception of goodness/heroism is in fact evil/unheroic. No amount of GM ranting about it being his/her world changes this fact that this applying alignment against a player's wishes is dismissing that player's own evaluative judgement.
Is it so much less insulting to have, when a cleric returns to their home church, the cleric hit over the head with a mace, tried and executed? Eventually, you're playing in the DM's world, and there will be consequences for your actions, unless you eschew NPCs you can't kill.
What about other characters? Are they bound not to mention that they think Eric the Red is scum? Or is that dismissing the player's evaluative judgment that his character is a good guy?
A GM should not use alignment as a straightjacket. But ultimately, alignment or no alignment, a player's evaluative judgment is going to be called into play by the NPCs, the other characters, the other players and the GM, who may well not be interested in running that type of game.