He claims prison gives him "unbearable" traumas
Isn't that kind of the point of prisons? He did the crime, he should do the time...simple as that really.
I suppose that it's a reasonable question to ask. What is the point of incarceration? I can think of at least three reasons why we might put people in jail. First, it removes them from the the public, therefore it gives us a sense of safety. Second, it may offer a chance to rehabilitate them. Third, it may be punitive. You seem to be focusing on the third reason. If that's really the reason for putting people in jail, then we can reasonably conclude that the best option is to let him stay in jail. If, on the other hand, we put them in jail merely to remove them so that the public need not fear them, then killing them seems like a less expensive option, and one with more finality. Then again, if the point of incarceration is the middle reason, rehabilitation, then neither traumatizing nor killing the convict serves the purpose. I suppose your answer to the question will inform your opinion of sentencing. There may also be other reasons for jailing people that I'm not thinking of at the moment.