He had a genocidal impulse toward Picard. His world, his perfect life. He wanted to take it all away. "My life means nothing while you're still alive." he told Picard. He also wanted to show Picard that what "he" was capable of, show him that underneath all the nobility and Federation ideals, was a man capable of unspeakable evil. "Look in the mirror, see yourself!"
As for the Romulans, he had no need to kill them, since he'd already conquered them. A lowly human slave had risen to rule those who mistreated him so badly.
The guy was nucking futs. Mentally ill. In his worldview it made sense.