Is it mandatory to kill someone for leaving Islam?

There is a similar question on this. It is a commonly known fact that Muhammad SAWS and many caliphs punished apostasy with death. However, my question is **whether it is *commanded* to punish someone with death for leaving Islam**. There are many Sunnahs which are not necessarily followed, like having a beard, or cleaning teeth with siwak. The Quran has direct commandments on how to punish some crimes, like theft and adultery. But is the death penalty for apostasy merely 'good practice' or is it a commandment?