That was the expected assumption…
Klein was supposed to be committed to the deal to "obtain" Luna, had accepted the conditions of unconditional support for the plans, was expected to let his group be incapacitated without real resistance or even help the attackers.
The arrangement had been clear. Klein's full cooperation in exchange for the arranged marriage that would restore Goldcrest prestige and give him access to Luna and the Heart.
Simple political transaction where everyone got what they wanted.
Except Klein was supposed to stay neutral here at minimum. Was supposed to let this happen without interference.
Then why?
They'd planned to knock out one of the kids in Ren's group and use him as hostage against Zhao. Force the professor to withdraw under threat of permanent harm to students.
Standard 'kidnapping as leverage' tactics. Exploit the teacher's obligation to protect his students, use that protection as the advantage to neutralize the most dangerous opponent.
