The Iraq Body Count website (
http://www.iraqbodycount.org/) is a database that compiles eyewitness accounts, medical reports, police reports, and US military reports. While it is by no means 100% infallible (and thus, not 100% accurate), it is likely to be a 'ballpark figure'.
'Common f'n sense' would dictate this as well, but it seems to have eluded you.
Just because the number is high (alarmingly high) doesn't mean it's untrue.
And as for your question of motive - I don't know, and I think that's the worst part of it all.