This is probably to sow distrust. The Demon King or whatever doesn't need anyone to accept, it only needs to offer. Because no one will know who accepted or if anyone did, so they'll be splintered regardless and probably result in actual backstabbing by people thinking they're betraying someone who wanted to kill them when in fact they'd all be victims.
Not all of the top ten were present or at least shown in the chapters or named. So, the problem is that they are having Paladin interrogate everyone which severely wastes time when there's truly only like three people that need questioning. The top five are present plus Sword Saint plus Kim. That's seven people. So, only only one out of three could be the traitor because of the voice-activated commands.
This, by the way, means the individual in question was probably offered the reward last and killed the other two immediately after receiving it, assuming they were even near each other.