I agree, but the shame of the matter is, they don't portray it in the film as being that big of a deal. Not only does Padme immediately forgive him, she marries him days later! Clearly Lucas didn't consider the Sandpeople people.
If he wanted to show Anakin gradually fall, doing something not necessarily evil but questionable, why not show him slaughter actual animals. For example, is mother is lost in a cave hunting mushrooms or something. Anakin goes searching for her, and finds she has been hiding for days after being attacked by some CGI alien bears or something like that. When Anakin shows up, she dies. The bears attack, and he loses control and slaughters every one he sees.
Later he reveals to Padme what happened and that what he did, even though it could conceivably be called self defense, was done out of anger. Anakin insists that he could have escaped, and thus wasn't justified in killing them. Padme comforts him, telling him might have died trying to escape.
Come to think of it, that would have worked just as well with Sandpeople if they didn't include the killing women and children part. Just that Anakin could have escaped but chose to stay and fight. The audience is left wondering if he really avoiding escaping to kill, or if he's simply beating himself up after the fact because he hates the idea of killing.
Instead of lots of grays, we are left with a clearly black and white "Anakin is evil now" situation.
