Well, I think you're being kind of childish, for starters.
You're also adding to the symbolic value of the thing you are indicating you hate -- i.e. the flag. You're assuming that Americans whose politics you disagree with are going to be almost worshipping the flag, treating it almost as a supernatural and sacred object rather than a piece of cloth, however symbolic the cloth may be. And by destroying that symbol, you're acting as if the symbol is almost as sacred as the patriots believe it is.
I wonder if this is the best way to start your revolutionary program, the way members of the Revolutionary Communist Program often try to launch their radical careers. I wonder if it wouldn't be better for you to do something less symbolic and more realistic -- like protesting a particular war or invasion or example of politic brutality, for instance. Flushing a flag down a toilet, in way, is kind of a nerdy thing to do, not a "real" one.
Are you "telling the truth" with this nerdy & symbolic action? Well, I think you can be, but I think the method you're using to deliver that "truth" may get in the way of other people hearing your message.
Personally, as an American who's been a teenaged libertarian super-patriot as well as an angry student radical, I think it's obvious that the American flag has been used to "justify" some horrible deeds. It's been used to provide excuses for crimes and atrocities that totally violate the moral values that most Americans think this country stands for.
When you first realize how many horrible things people have done in America's name, it's tempting to want to destroy the flag in order to express your anger and your moral outrage. But it doesn't work; it doesn't change anything. And from my personal experience, I think it often is just a way of trying to shock your parents, as part of the painful experience of growing up.
If you're furious enough at the hypocrisies and the lies of the US government to want to stage a melodramatic protest -- well, that may be good. It shows that you're thinking. But if you flush a flag down a toilet as you're suggesting here, you're just going to convince many people that you're childish and a bit immature and stupid. "You're just acting out in rebellion against your parents," they're going to think.
You may believe you're telling the truth AND being unpatriotic, and you may feel you're committing some kind of bold revolutionary deed with your flag flushing. But most people aren't going to understand the "truth" you think you're telling, and so I think this is mostly useless.
-- democratic socialist for truth and common sense