I know it's just "Finish him" in Mortal Kombat, but is it really grammatically correct when, say, writing dialogues for characters in a story (eg. a villain saying the line in a superhero story)? All dictionaries say it's "finish him off."
Grammatically "Finish him" is an imperative verb with a direct object, and is completely normal: like "Eat it".
Lexically, "finish" meaning "kill" is less common than "finish off"; but it does occur.
The iWeb corpus has 2060 instances of "finish/es/ed him off" and 3323 of "finish/es/ed him" (including the 2060).