[dammage79]

There was the purge of the Jedi temple for that Saber, then there was Mace Windu, the beheading of Dooku when he should have been arrested (Anakin knew full well the proper action and gave into the anger.) The masacre on Mustafar followed immediately by the battle with Obi-Wan. The Sabre was in Anakins possession when he killed (loosely using the term here, he didnt kill her on the spot but the action removed her will to live) Padme as well. There's a lot more Hate, Anger, Fear in that Saber than we're covering, all those events were really heavy in Anakins turn to the dark side. Then add to that the way Rey took down Ren in anger. Yeah it was anger, I think that was clear as day. whether or not that was from the connection with the saber itself and Rey gave in to it's "will" or if it was just an untrained Jedi not knowing how to control herself, it was still anger.

As for the how, I've seen a couple of references where Rey has a force ability called Phsycometry that allows her to see any items history by touching it. I couldn't image dealing with that much of an emotional load coursing through my mind and not giving into it at least a little. I also think thats why Luke looked so sad when he saw that thing. Like he knew full well what that thing was. A heavily tainted relic burdened with the atrocities of the past. A damning mark on the Skywalker name.