well, time to update this post.
this past week, I got 2 Legend Slimes, Aether and Rain. in the Codex, Rain is shown with having Bad Skill, while many other slimes have Boost instead. now that I have Legend slimes with both in the Codex, I now know that the Bad Skill in the Codex that is applied to the Legend Slimes is actually the Boost that is shown on the other Legend Slimes. I guess that when slimes were added to the Codex, there were a few mistakes made, and that is how the Bad Skill Secret Society was formed.
many thanks for every one who helped me with so much good information.