Was it just a coincidence that he wanted them to become strong just after the knights arrived? Maybe. But he did not feel that that was the case.
It meant that the king wanted protection against the knights that had arrived. That was what he had concluded at first, but that did not make sense after he thought about it.
True, the king may be evil, and would do anything to achieve his goals, but he had still not done anything 'evil' yet, had he?
Then why was he afraid of the knights? All he had to do was adjust his conduct a bit and he would not be harmed by them in the near future.
Unless... he wanted to gain control over them.
They followed the words of Robinson religiously. Their loyalty to the king was inferior to the loyalty they had towards Robinson.
Arnold pretty much understood what was happening.
When he was given the opportunity to gain power with the 'injectable', he had already decided that he would use it.
