Unrestrained
'This makes more sense.'
If anyone could transform the nature of their Internal Energy to that of 'Demon Sealing' then rather than a newbie like Yeon, someone like the Absolute Spear would have been far more fit to become the new Heavenly Demon.
'I'm not a protagonist after all.'
Attempting something impossible for normal people only to find that it actually worked out for some mysterious reason was something exclusive to the main character's enjoyment.
In reality, Leo only got his hopes up because his perception gave him a false sense of grand expectations.
That being said, he was still delighted to see that Yeon's transformation was successful so far. At the very least, it didn't seem like his involvement had made her path to power deviate from its decided course.
Besides…
'I did get exactly what I wanted.'
Even though the sense of disappointment lingered in Leo's mind by the premature end of his transformation, something else was willing inside him.