The end justifies the means
The end justifies the means is a phrase of Sergey Nechayev, the 19th century Russian revolutionary. It means that if a goal is morally important enough, any method of getting it is acceptable. It was later picked up and used to justify murderous acts by Lenin and Stalin.
The idea is ancient, but it was not meant to justify unnecessary cruelty. It was part of a political philosophy called consequentialism. The basic idea is that a policy can be judged by its outcome. All modern versions of consequentialism have to deal with the limitations necessary to prevent tyrants abusing this sensible idea.