@fresco,
In general, the first obligation logically takes precedence since there are alternative sentences to support the second.
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Sorry your logic have a few holes in it..
First you are at least adding to the risk of death to prison guards and other inmates as lifers under a no capital punishment had little to loss by taking the life of a guard or another inmate.
Second there had been and more then likely in the future will be prison break outs of murderers so warehousing them add slightly to the general population risk.
By allowing mass murderers to keep living you are reducing society respect for it laws and in the case of terrorists you are adding to the general population risk that followers/supporters will seize hostages to force the government to released them.
This is not a theory out of a novel as it had happen once more I refer you to the West German problem in that regard that occuring the 1980s.