Why in Sharia Law are people allowed to kill?

Sharia law fully compliant with the Quran virtually eliminate the capital punishment, in fact even in case of a murder, the final say is given to the relatives of the victim and rules of blood money ensure that the victims are not left on the street because the bread winner is killed. Besides the standard of evidence applied is much more strict before somebody would be sentenced to death. Single witness conviction are non existent in Islam, in fact you will need four upright witnesses before the maximum punishment can even be considered.