@DeviLaugh
It was stated that the Empress drank the poison meant for the Emperor, but nothing beyond that. So someone tried to poison Leon, but somehow Lizzie drank it instead. Considering how dead set Lizzie was on being the perfect Empress, its entirely possible she tested things meant for the Emperor to see if they were safe. But I'd like to think he wasn't seeking her death. It said in chapter one of both the novel and the webtoon that Leon was "indifferent" to Lizzie, so I don't think he hated her enough to kill her. He just didn't care what happened to her, live or die.
That could also account for his shift in personality happening so easily, as going from indifference to interest to regret isn't like going from hate to love. Even now after the time reversal, I don't think he quite loves Lizzie just yet. He just regrets how things turned out and wants to help her, and will grow to love her in their new future.