In a civilized society there are accepted norms of behavior, without such chaos would ensue. When a person commits the ultimate sin, that of unwarranted murder, they have proven that they have no regard for the rule of law. Murder that is committed against anyone is horrendous and a tear against the moral fabric of society, with no redeeming qualities.
As such, a civilized society, upon conviction should be allowed to remove the offender from the society which they have no regard for.
In a world where a person who commits homicide, and is allowed to live at taxpayers expense is a travesty against society. The United States spends a huge amount on incarceration and all of the benefits that society deems required of inmates.