I like the idea of Anakin dreaming (fantasising) about killing the sand peple.

This is off the top of my head, and probably crazy: what about if he thinks he dreamt it - due to Palpatine's dark side powers combined with Anakin's subconscious suppressing it - and right after Anakin kills Mace, Palpatine lets the truth hits him: that's he's a mass murderer. You could then add a montage showing Anakin feeling the pain he inflicted on the sand people, followed by Anakin's "What have I done?"