وَقَتْلُ النَّفْسِ means “and killing a person”; this context warns against unlawful killing. Here النَّفْسِ means a human life, whereas نَفْسَهُ earlier referred to the self. Context controls the sense.
In وَالْيَمِينُ الْغَمُوسُ, الْيَمِينُ means “oath,” and الْغَمُوسُ means “plunging” or “immersing”: a deliberately false oath that plunges a person into sin. The image emphasizes gravity.
Example: someone knowingly swears falsely to conceal what he did. A deliberate false oath is grave even when no money was taken.