That yakuza boss ended up getting assassinated.
Have you already watched the Assassination Classroom movie? It was fun, isn't it?
Hey, Viet! How did you come up with such a unique assassination plan?
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.