Well, it would definitely cause problems, but Damien never planned on actually killing the Dragon Emperor.
Still, he wasn't going to let the man go if he was truly a disappointment. In that case, there were far worse things he could do besides just killing.
At the end of the day, the Dragon Emperor was too important of an entity to all dragons. Even if he was just a figurehead, his life was still valuable. Even if all of the Holy Clans fell, the dragons wouldn't lose hope until their Emperor was taken as well.
Damien had a certain level of respect for anyone who held the title. It wasn't an easy one to bear, especially for the more recent Dragon Emperors.
At the very least, the heir wars proved that he was still trying to influence the kingdom.
Or, he had given up and was looking for someone to take the burden off of his shoulders.
The current Dragon Emperor was a so-called "Aura Dragon."
