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

Allah [The Exalted] says:

مَنْ قَتَلَ نَفْسًا بِغَيْرِ نَفْسٍ أَوْ فَسَادٍ فِي الْأَرْضِ فَكَأَنَّمَا قَتَلَ النَّاسَ جَمِيعًا وَمَنْ أَحْيَاهَا فَكَأَنَّمَا أَحْيَا النَّاسَ جَمِيعًا

If anyone killed a person not in retaliation of murder, or (and) to spread mischief in the land – it would be as if he killed all mankind, and if anyone saved a life, it would be as if he saved the life of all mankind. [Surah Al-Maa’idah. Ayah 32]

Meaning: The killer lacks any valid reason to take a life, and it is imperative that no one should kill another person unless there is a justified cause.

If an individual is audacious enough to murder someone who does not deserve it, this indicates that they do not differentiate between the person they killed and others, rather, they would commit this act whenever their evil inclinations prompt them to do so. Consequently, their audacious behavior, which led them to kill, is tantamount to killing all of humanity. Conversely, if a person refrains from killing and instead saves a life, even though their inner desires urge them to do otherwise, out of fear of Allah, it is as if they have saved the lives of all humankind. This is because the fear of Allah that they possess prevents them from killing someone who does not deserve it.

An Excerpt from Tafseer As-Sadi. Slightly paraphrased