Everyone has the right to life, liberty and security of person.
Now, obviously we as a society cannot always guarantee these three things for all people, since criminals infringe on the rights of others and thus human rights are violated daily. In order to prevent this we choose to infringe on the liberty of criminals (by imprisoning them), in order to prevent their (greater) infringement on others. But since it is possible this way to prevent their infringement, by limiting only their liberty, there is no way to justify killing them instead, taking away not just some, but all of their liberty along with their life and their security. Yes, in order to enforce the human rights we must (somewhat ironically) infringe on them ourselves, but we cannot justify performing a bigger infringement than is absolutely necessary.