In The Name of Allah, The Most Merciful, The Bestower of Mercy.

The Silent Admission of Willing (Intentional) Aware, Rational Rule-Breaking Murderers Without Evidence of a Legally Defined Self-Defense Case When the Strong Influence of Taqwah Is Absent to Restrain the Wicked Soul

Imam As-Sadi, may Allah have mercy upon him, stated in his Tafsir of Ayah 32 in Surah An-Nisaa about the one who kills without a justified reason:

فلما تجرأ على قتل النفس التي لم تستحق القتل علم أنه لا فرق عنده بين هذا المقتول وبين غيره، وإنما ذلك بحسب ما تدعوه إليه نفسه الأمارة بالسوء. فتجرؤه على قتله، كأنه قتل الناس جميعا. وكذلك من أحيا نفسا أي: استبقى أحدا، فلم يقتله مع دعاء نفسه له إلى قتله، فمنعه خوف الله تعالى من قتله، فهذا كأنه أحيا الناس جميعا، لأن ما معه من الخوف يمنعه من قتل من لا يستحق القتل

When he had the effrontery to kill a soul that does not deserve to be killed, it is known that (or it implies that) for him (i.e. the murderer) there is no distinction between this one who is killed and other than him (i.e. another person not murdered yet), rather,  that (i.e. his deed of unlawful killing) is due to what his evil soul calls him to. Thus, his dareness to kill him is as if he has killed all humankind.

On the other hand, the one who saves a life, meaning, spares the life of someone and does not kill him, while his soul entices him, but fear of Allah, The Most High, prevents him from killing, this is as if he has saved the lives of all humankind, because the fear (of Allah) with him prevents him from killing the one who does not deserve to be killed.

An Excerpt from “Tafsir As-Sadi”. Paraphrased

NB: In all of this, it’s important to remember certain points that are present in the final divine revelation and in the different Tafaaseer of the Qur’an, including Tafsir As-Sadi.

The murderer is dealt with along with the rights of the victim and their family through legal means. Vigilantism is not allowed under any circumstances.

The doors of repentance are still open for the murderer even though the legal consequences are still in place from the beginning to the end through the courts.

All of this is outlined in Shariah law regarding crimes.