If according to Dazai words, "Chuuya wants to kill me out of hate but he has to protect me because I'm the boss." It's hurt, isn't it??
I mean, Dazai can do anything to himself just to see Chuuya is frustrated because he was injured. Or at other times Dazai would being jerk on Chuuya, but Chuuya couldn't fight his boss. Chuuya is a dog. Faithfully blind.
Dazai isn't as patient as Mori, waiting for people to reach the point of use, he can just throw them away if they can't be used. Just imagine the executive must have a subordinate right? Like Dazai has Akutagawa or Kouyo has Kyoka & Chuuya then. Maybe Beast!Chuuya also had friends or subordinates. Because Chuuya is a real dog, he loves everyone close to him. Imagine the day his friends or subordinates do wrong and get kicked out by Dazai. Killed is the worst.
Or maybe just because Dazai wanted to torture Chuuya, he made a plan so Chuuya's subordinates/friends were accused of being traitors and excecuted using mafia procedures.
Then, since he's smart, Chuuya knew that it was all Dazai's plan.
You could say this like main story, Chuuya in Main!Dazai's position who lost Odasaku because Mori's plan and Chuuya here lost his beloved friends/subordinates because Beast!Dazai's plan.
But not as lucky as Dazai in main story who could get out of PM and then go into ADA, Chuuya, in Beast was really unable to betray. "Not in my blood" is the term.
So he swallowed that bitter reality deeply, and was still loyal to Dazai. Really a dog he is. T_T
But, the truth is Dazai won't be that bad. Because this is HeadCanon, not FIC, so the character will fit the story. And in the story, Dazai won't be that bad with Chuuya because no matter what and which the story is, Chuuya is still his partner.