I find some truth in the saying; what doesn't kill you will strengthen you, and it is especially true in cases where one truly feels the impact of the situation and then improves themselves so that they do not have to experience the same situation again. Or if one truly feels regretful of their actions, then they will do what they can to avoid putting themselves in the same situation again. In the sense of self-improvement, to become better, to handle a similar situation better, to avoid being and having the same result of a previous experience/situation, then this saying probably has some truth in it. In most cases, it is because you allow your mind to become stronger and better.
BUT!.. say if you were a burglar, and you keep succeeding in your breaking-ins and avoid getting caught. It will strengthen the burglar's bravery and courage to keep carrying out that act. Of course karma comes into play, but the statment would also be true for this.
So, the saying works, but again it depends on ur motivation, what you want to strengthen - yourself, your evil deeds, your malicious intents, your achievments, your attitude and goals... it's your choice.