The Tuskens deserved it

It's never stated explicitly, because AoTC was a movie with children in the audience, but the Tuskens were most likely SAing Anakin's mother the whole time she was captured by them. Why else were they keeping her alive, tied up to a rack? The alternative is that they were just torturing her, which is arguably the same/worse. And that's just what they do to people they don't murder outright.

A culture that does these things and considers them normal should not be allowed to exist. Each dead Tusken, including the young, was who knows how many lives saved, or women spared from a similar fate as his mother. Anakin did Tatooine a favor.