FormerlyHemlock
Hero
In Barbarian legend in character, Barbarians attack more savagely and ignore some of the shock of wounds while doing so.
But how does the Rakshasa know that they ignore more wounds than they do more damage?
In this case, it doesn't matter. "Hit yourself as hard as possible" might cause the barbarian to rage, but since he'll be hitting himself with a magic weapon, rage doesn't confer resistance anyway. He's not bear totem, he's wolf.
A command "kill yourself as quickly as possible" will leverage the barbarian's own knowledge, maybe even the player's knowledge. The player knows whether raging will speed up his suicide or not.