Assassination
noun, verbal noun
A dark kill is an assassination. It's done by someone who is in a ninja outfit and dark, hiding in the shadows for his moment to strike.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
そのヤクザの組長は暗殺されてしまいました。
That yakuza boss ended up getting assassinated.
もう暗殺教室の映画は見ましたか？
Have you already watched the Assassination Classroom movie?
ねぇ、ビエト！どうやってこんなに面白い暗殺計画を思いついたの？
Hey, Viet! How did you come up with such a unique assassination plan?