Why do muslims think that every other religion is evil and only islam is the right way to go?

There are extremists in every religion. People use it to justify stupid things MANY times throughout history. One of my best friends ever is a Muslim (albeit not a very outspoken one) and I honestly chalk it up to stupidity on certain people's parts, not on the religion as a whole. And you know what? Rawr. AFAIK (and I could be wrong,) Islam teaches a "scale" of good and evil deeds and if the good outweighs the bad, you're pretty much good. Obviously following the religion is considered a good thing, but I'm almost positive that good people of other religions can end up in Heaven according to Muslim beliefs.