It’s the only deal he’s ever broken and even though we know he can break the curse with “True Love’s Kiss”, after centuries of living I don’t think he wants people like Regina to go unchecked. He wants to end all magic, eventually.
That’s been my theory through most of the first season. I really believed that ending magic was his ultimate goal with this curse. That’s why it threw me when he was so eagerly bringing it back in the finale. Guess this could just be another step in is plan though, and that is still part of the ending he wants.