"Boy, do I HATE the cool types. Cool types are so atrocious - they don't care if you're hurt, attack anything they don't like, and spread nasty lies behind your back. They look at me like some...some kind of weird THING and criticize every little flaw - LAUGHING all the while! 'You go to royal dances? What a wimp!' 'Reading is so not you!' 'But you should be cool!' 'You don't act very cool!' 'Why can't you be more extreme?!' What does it mean to be 'cool'? What does it mean to be 'extreme'?!"
Well to me her shadow would probably say something like this: "The reason you want to join the wonderbolts? You just want to be popular. Face it, you don't talk about how amazing you are all the time because you mean it, you just want to show confidence so ponys respect you. You're nothing without other ponies. You can't stand to be alone that's all. Those, so called friends of yours? Nothing more than crutches to keep you sane. You don't care what happens to them because you like them. You're only worried about losing a source of encouragement. Your element is loyalty? Please, don't make me laugh! The only pony you're loyal to is yourself!"
Remember the shadows aren't cliche Hannibal lectures, THEY REALLY ARE what the ponies thinks and feels in the deepest most forbidden parts of their minds, the thoughts and feels they never want others to know about, nor do they want to acknowledge of themselves.
Actually, what you say is half true. Shadows represent, basically, the darkness in their heart. So while they do reveal their deepest darkest secrets, or true self. They also say things that aren't true, like Kanji being scared of girls, or Youske not liking Saki. They do it to make the original not except them, so they can become a different person. Due to this, they are more or less, twisted doppelgangers.
He admits to the part about using Saki as an excuse to go in and become a "Hero". But his Shadow goes on to say he doesn't give a shit about her or anyone else, which is obviously wrong from the way he acts before she dies.
(okay was going to respond to this a few weeks ago, older drawing so I completely forgot) The shadow is supposed to be the repressed part of the subconscious, so deep down rainbow doesn't see herself as evil, she sees herself as being awesome and good enough to be the leader of the wonderbolts.