Lucas didn't know every rank there was, but from what he already gathered he could put a simple ranking system together. The ranks that he knew of in order of importance:
Nobility :
King/Queen
???
Viscount
Baron - Mage 1st circle
Common titles :
Knight
Soldier
Commoner
With every title of nobility standing above the common titles. Lucas didn't know how to obtain those titles. Those were troubles for later, first, he had to escape from the baron and Venion. He had a plan, but it wasn't finished yet, there was a multitude of things he needed to check first. Like what the limits of the contract were, how it actually works, and so on. If he wanted to just escape right now, he would need to kill all members of the baron's bloodline and Venion on top of that. Doing all that without leaving a trace of his existence, if there were any clues at all, some government forces would eventually find out, tracing it back to him. It would take time, but it wouldn't be a surprise according to part of Lucas' mind.
But if he could do so without leaving a trail of evidence of his existence, he would be a man without an identity in this world. No one knew him, he was an orphan that was left behind at a young age, he didn't have any standing, and he was a nobody. This was good and bad at the same time, he would need to forge his own identity later on without making it seem unbelievable. But before he bothered to think about that he first needed to escape this mansion, which resulted in him killing someone either way.
The contract needed to be destroyed, so the baron needed to cancel it, threatening him wouldn't work if he could simply call Venion, killing him was a lot easier. Assassinating him was the only option since he could force Lucas to simply never attack him. Even if he was killed, all his relatives could command him to their will. The mansion needed to be rooted out and he needed to hide until he was strong enough to break the contract himself.
In reality, there are more ranks than the ones stated above, there are also counts and marquis. This ranking system is applicable to the country of The Holy Kingdom, but neighboring countries rather use their own system. It has been ingrained into society that ones of a higher rank look down upon the lower ranks. There are people who want to change this system, but their voice is a minority.
Per rank there are certain advantages you can get from the Royals like mining rights, land to develop or not having to pay any taxes. Religious groups have their own system and are somewhat special. The Order of Light rules The Holy Kingdom for instance. Their head is called the Lady of Light, rumored to be the offspring of the Goddess of Light.
Other countries have a split between the rulers and religion, still the Order of Light is everywhere.