Speaking of mass murderers, mass murderers should be freed from prison on the condition that they join the military. Part of being part of the military is knowing how to use a weapon with confidence and without fear or queasiness. To be a soldier, you need to be able to kill opponents. Mass murderers are not only not scared of killing, but they also enjoy it. They have experience that would make them good soldiers. Also, many mass murderers have actually already been trained by the military. There should be more wars so that those with killer instincts have a legal outlet for their desire to kill. Then, instead of killing civilians of the same country, they'll join the military and do well at helping win the war. It's a waste of skill to not hire them into the military.
Serial killers, too, are useful to society. Some of them have very high IQs. For example, Google says of a serial killer: "It's widely reported that [Ted] Kaczynski scored a 167 on an IQ test in fifth grade." It's a waste of genius to have "another Einstein," as he was referred to, in prison. He should be free and contributing to society by being a mathematician professor which was what he was before he got arrested according to Wikipedia. That's just one example. There's a list of high IQ serial killers.