wa alaykum salam
about 4:93, what I think when I read this ayah is that the believers have a really special status for Allah (swt), he separately mentions and emphasizes the punishment for killing one of them. (Though it's just my interpretation and Allah (swt) knows best.)
"Is one who worships devoutly during the hour of the night prostrating himself or standing (in adoration), who takes heed of the Hereafter, and who places his hope in the Mercy of his Lord - (like one who does not)? Say: "Are those equal, those who know and those who do not know? It is those who are endued with understanding that receive admonition." (39:9)
Without a doubt, Allah(swt) is Al-Rahman, the most merciful. He is the protector of all human beings! So, a general punishment for killing an innocent human is not just restricted to "believers".
"Because of that, We decreed upon the Children of Israel that whoever kills a soul unless for a soul or for corruption [done] in the land - it is as if he had slain mankind entirely. And whoever saves one - it is as if he had saved mankind entirely. And our messengers had certainly come to them with clear proofs. Then indeed many of them, [even] after that, throughout the land, were transgressors." (5:32)