There are other gem kings with the power of the C grade, but the two of them are currently the strongest gem kings, which makes them the the strongest manpower that the gem civilization is capable of mobilizing.
The Gem people are not a race that is inherently violent. They like sleeping and can be peaceful. But once the fight for precious material comes up, the Gem people become capable of violence.
After they get what they want, they usually return to their peaceful nature. This is clear by the fact that they haven't tried to kill anyone else after they killed the Snow King.
The gem people were content with gaining the support of the Sun King for mining Aries and transporting the precious materials back to their world. They were ready to bury the hatchet of war and returning to their peaceful lifestyle.
But now that there is someone trying to stop them from gaining more precious stones and materials, they don't mind sharpening their weapons for battle. The two of them had made up their minds to fight when the Sun King came to find them.
The Sun King asked them, "What should be done about the invaders?"
The red gem king was quick to reply, saying, "Of course, we beat them up and turn them into slaves for mining our minerals."
The green gem king replied in a roundabout manner, "Whatever reason they came here, they will have to return or help us achieve our goals."
He didn't say that he would fight the new invaders. But fighting is the only way to force the new invaders to return to where they came from or work for the gem kings.
Their replies made the Sun King sigh in relief and say, "I'm relieved that you all think like this. It is good to know that I am not alone in resisting the new invaders, or I would be contemplating giving them mining rights."
The moment the two gem kings heard of the possibility of their mining rights being given away to the new invaders, they became displeased with the Sun King.
The red gem king snorted in anger and said, "Why would you say something like that?"
The Sun King smiled and said humbly, "Because I don't want to fight the invaders. I am too afraid to fight them after witnessing your power."
Now, both gem kings became angry with the Sun King. This was because not only had the Sun King said that he was considering giving their benefits to the new invaders, but the Sun King also said that the new invaders might be equal in strength to the gem kings.
As if that wasn't bad enough, the Sun Kong continued to say offensive things. The Sun King said, "Now that we are working together, we will surely be able to defeat the new invaders even if they are slightly stronger than you guys, and I won't have to share the mining rights with them."
In order to eradicate this disrespectful presumption that the new invaders can match them, the green gem king said with displeasure to the Sun King, "Don't worry, your highness. We, two gem kings, are more than enough to defeat the invaders and send them running back home."
He had used the title of highness to speak to the Sun King, but there was nothing respectful about his tone of voice.
The red gem king was even more disrespectful as he sneered and said, "Yeah. Don't worry about the new invaders. We will take care of them."
The Sun King tried to dissuade them by saying, "Let me help you. No matter the strength of the new invaders, we will have a greater chance of defeating them if we work together."
But the two of them didn't listen to him. They nodded, said their goodbyes, and left. Their destination was the North Pole.
After they were gone, he didn't choose to follow them. Instead of doing that, he sneered and thought to himself, "Pompous fools. I hope they hit a hard wall there in the North Pole and are killed."
He is dissatisfied with the gem kings. This is why he didn't follow them to help them after they disregarded his opinion.
The truth is that he didn't want to help them fight the new invaders at all. He had said all of those things to provoke the two gem kings and make them risk their lives for him.
His feelings for the gem kings didn't start recently. He has been dissatisfied with them for a long time.
He didn't start out dissatisfied with them. He started grateful to them for helping them secure the throne of Sovereign of the realm. After all, without their help, he would have died before the previous Snow Kong grew too old and died.
But after he came the Sun King, he realized that he didn't like the feeling of sharing the ultimate power in the realm and living in the same house with people who could kill him.
Because of the presence of the gem kings, he couldn't do whatever he wanted. He always had to make sure that his decisions didn't offend them, or they would scold him or even threaten to replace him as Sovereign.
This was a bad thing to endure, especially since he had to stand by and watch the gem kings exploit his world for the resources that his people needed to grow stronger.
The resources are rightfully his as the Sovereign of the realm, and the people of his race should benefit from him being the Sovereign, but this is not the case. Instead of being the Sovereign whose words decided the fate of everyone in the realm, he became the caretaker of the mine for the gem people.
So he doesn't like the gem kings. He even hates them enough to wish that they were dead.
But of course, he doesn't really wish for the gem kings to be killed by the new invaders. He was only venting his displeasure when he made that wish.