Well, sometimes the only way to ensure that someone stops killing others for good is to end that person's life. Sometimes inaction leads to greater harm than action. If a person isn't willing to stop killing on their own, some force against that person is required to protect others by stopping that person from harming others. The same rationale is often used in criminal justice systems.It's not to prove anything but to punish the perpetrator of an unpardonable crime and to rid society of that individual. Interesting is the lack of understanding of many individuals as to the purpose of the penal system which is to punish. Rehabilitation is a secondary function in which not all are deemed worthy.

Oh!!!! Whom did u kill!!!!!

To prevent them from killing more (and that more can be me and you also).

BTW, Illyas Kasniri deserved it !