And yet, regardless of alignment, people will still play their character as they see fit. Being LG doesn't mean that a character has to behave that way all the time, nor does it mean he can't become something else (even if it seems to be entirely out of character; people *do* change for seemingly no reason sometimes).
And, if being tagged as something else, it is his actions that tell the Powers That Be what he ends up as. The character has no say at all as to what his alignment is; indeed, the most vile, Evil person can believe that he his LG, even act in the name of Good (ie, no eating babies or oppressing and such), but because of his actions (torturing, no mercy, genocide), is still Evil. Other people may not see it that way (those under his leadership may think him a saint!), but alignmentwise, he is still an Evil person.
The character is intended to come before alignment, and alignment is a largely powerless tag slapped on characters because of what they do, and sometimes, why they do it. Characters are always supposed to simply act; alignment is not really supposed to dictate what a character does, or thinks.