While we know he slaughtered the Tusken, but there was at least the distance between species (he was a human, they an alien species to him) and some of them had actively done wrong by him. I'm not justifying what he did, but I think it makes more sense for him to go ballistic mode here than for him to all of a sudden turn around in 2-3 days time from the beginning of Ep. 3 during the Chancellor rescue when he's with Obi-Wan, reflecting the essence of his character we saw so much of in Clone Wars, to jumping to him turning dark and then going on a Jedi children mass murder hunting spree.
Anakin was in a dark place during Ep. 3, but I think with the way the narrative is constructed, we are led to believe one of two things:
The Dark Side/Sith religion is just so, so corruptible to where once someone merely gets that small taste of it (Anakin killing Windu, accepting what he's done), they can just turn absolute psycho with the loss of any moral/ethic code they had left in the blink of an eye.
Anakin's mental capacity was literally broken beyond repair and he's functionally forgotten completely about being the man he once was.
I think if we saw Anakin actually push back a little against Palpatine about it but ultimately yielded after Palpatine convinced him, it would settle better with me that he turns such quickly into murdering innocents like that.