To start off, 'overkill' basically means 'to do something excessively'.
In gaming terms, it's like your enemy is already dead, but you keep using your ultimate power, giving the enemy no peace even in death, thus overkill. That's when people say, "Overkill la wei!"
The word 'overkill' can also be used in education. For example, when a teacher asks a student where in the History Book does it state that Malaya was under British Colony, the student replies, "Chapter 4, Form 3 and Form 4 textbook, page 40 and 60 respectively, paragraphs 3 and 6, lines 4 and 16." Mind you, that's when the teacher says, "Overkill la wei!"
Well, if you're still not convinced, 'overkill' moments happen in the Bible as well. 2 Kings 9:32-37(NIV) says:
He looked up at the window and called out, "Who is on my side? Who?" Two or three eunuchs looked down at him. "Throw her down!" Jehu said. So they threw her down, and some of her blood spattered the wall and the horses as they trampled her underfoot.
Jehu went in and ate and drank. "Take care of that cursed woman," he said, "and bury her, for she was a king's daughter." But when they went out to bury her, they found nothing except her skull, her feet and her hands.
They went back and told Jehu, who said, "This is the word of the LORD that he spoke through his servant Elijah the Tishbite: On the plot of ground at Jezreel dogs will devour Jezebel's flesh. Jezebel's body will be like refuse on the ground in the plot at Jezreel, so that no one will be able to say, 'This is Jezebel.' "