The dictatorship was responsible for the lives of those it had unjustly sentenced to death.
The dictatorship was responsible in the lives of those it had unjustly sentenced to death.
The dictatorship was responsible for the lives of those it had unjustly sentenced to death.
The dictatorship was responsible in the lives of those it had unjustly sentenced to death.
When discussing the issue or object of responsibility, the adjective "responsible" idiomatically takes the preposition "for."