What happens if I become an illegal target of Cruel Entertainment?
Nothing happens at all as the spell resolves. Neither player controls the other.

Under 608: Resolving spells and abilities comes these rules:


  608.2b If the spell or ability specifies targets, it checks whether the targets are still legal. [...] If the spell or ability is not countered, it will resolve normally. Illegal targets, if any, won’t be affected by parts of a resolving spell’s effect for which they’re illegal. Other parts of the effect for which those targets are not illegal may still affect them. If the spell or ability creates any continuous effects that affect game rules (see rule 613.10), those effects don’t apply to illegal targets.


Since one of the players is an illegal target:


that player can't be made to control the other player.
that player can't be made to be controlled by the other player.


Both players retain control of their own turns as normal and the spell has no actual effect on anything.