The problem is that regardless of the dislike myself and others might feel towards Mr Roof, it is not right to subject him to a punishment without going through the correct legal process.
I hold the view that Mr Roof should be kept alive and well so that society in general can deal with him at a court and then the prison service in the US may well have to carry out the instructions of the court. It is interesting to note that Adolf Eichmann was kept in jail with great care to prevent him killing himself and also a great effort was made to keep anyone who would have a reason to take revenge on him away from him. This was to keep him alive so that the court imposed sentence could be carried out. For details see http://www.aish.com/ho/i/48959761.html
Mr Roof could be subject to the death penalty, even if he is he should be kept alive and protected from informal punishments. If he gets a jail sentence then he should be kept alive and well, while he is known to have killed black people at a church I think that a lot of people of all races would find him to be a very nasty person, I imagine that some people (of various races) might want to beat him up or even kill him in jail. This in some ways would be allowing him an easy way out, my own view is that he should be kept alive to face his punishment in full, that is assuming that the trial comes out with a guilty verdict. I think that it is going to be very likely that he will be found guilty of murder.
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Health is a state of physical, mental and social wellbeing and not merely the absence of disease or infirmity

I am not a jigsaw, I am a free man !
Diagnosed under the DSM5 rules with autism spectrum disorder, under DSM4 psychologist said would have been AS (299.80) but I suspect that I am somewhere between 299.80 and 299.00 (Autism) under DSM4.