I see the evil Mentalism version:
"Target becomes guilty over some incident in his past. He will not perform such an action again, and must take steps to overcome the guilt (rectifying the past wrong)."
I would focus on the nature of the incident, what aspect does he now believe to have been wrong? E.g. was he negotiating unilaterally to the detriment of the party? If so, is it that he is unwilling to negotiate at all ever (which seems a very broad reading) or for example does he feel a need to involve the other party members in any such negotiation? There's considerable room for interesting roleplay here, it should be something that can be used and not simply something that excludes. And, what amends does he feel are required?
And the Sorcerer version:
"Target becomes guilty over some incident in his past. The GM should choose some significant event in the target's past. He will not perform similar actions again and must take steps to alleviate the guilt."
Good luck with that. With an evil character, the odds of targeting the negotiation are low. The chances are much better that it will target some evil thing the whole party was involved with, and personally I would be inclined to make this PvP mind control something the caster comes to regret so it doesn't recur.
I don't see a purely Essence version but I might be overlooking it. If you are using RM2/RMC, I don't know which one the spell resembles more.
That said, I also would want to examine the party dynamics. If you have an evil party that is using mind control against each other, you are going to have some vicious infighting and loss of agency. Are those things your players will enjoy? If so, great, proceed. But a lot of players will not and you are better off nipping it in the bud.