Kaizen knew that if he chose to unlock the Class Skill <Soul Control>, it would consequently take longer for him to be able to unlock <Sidereal Fusion>, which had the function of fusing different items together.
Both skills would be extremely useful for him, because one would enable him to take the souls of evil spirits, while the other could allow him to put together, for example, two different swords, creating a single sword. Whichever of these two abilities he unlocked would also be a complete gamble because he couldn't tell what their limits were.
In the end, he decided to try to unlock the Class Skill <Soul Control> first and then focus on the <Sidereal Fusion>, because the former was the one he was most curious about precisely because it had a contextual connection to the last Class Quest he did.
While repairing the armor he wore in the battle against Taznaar, Kaizen was thinking seriously about these matters.