Wait you mean Darth Vader who supposedly personified evil created a evil heinous act as he fell to the dark side? Why is that unexplained? The dark side took him over. There was no good left in Anakin at that point. Isn't that the point to see how evil Vader was early on and to see him commit acts of evil?
Well last I checked, the dark side isn't some evil spirit like in the Exorcist that suddenly possesses you so you can no longer control your actions. Even as twisted and corrupt as Anakin had become, it's still a pretty huge leap from him being upset with the Jedi for not making him a Jedi Master and concerned over Padme's life to... suddenly deciding to murder everyone in sight on Palpatine's orders.
Especially given that only a few minutes earlier he was perceptive enough to see through Palpatine's deception and turned him into the Jedi. But then suddenly he decides "never mind, Palpatine was totally right and all the Jedi need to be killed now?" Say huh?
Psychologically speaking, nothing about his final fall to the dark side in that movie makes any sense. At least to me anyway.