Maxim #37 is "There is no "overkill". There is only "open fire" and "I need to reload." What does this mean?
Though perhaps better suited to English Language the clear source gives us a fig leaf of coverage...
Webster's defines overkill as;
1 : a destructive capacity greatly exceeding that required for a given target.
2 : an excess of something (such as a quantity or an action) beyond what is required or suitable for a particular purpose publicity overkill an overkill in weaponry.
3 : killing in excess of what is intended or required.
Maxim 37 means that maximally effective mercenaries do not believe that there is such a thing as too much destruction. Any and all levels of force are acceptable in the pursuit of the goal.
The second part of the quote is an example of overkill. Keeping firing even when the target is already stopped would usually be considered overkill, the mercenaries however believe in continuing firing until they need to reload.
It's an old joke about how people in movies shoot the monster once, it falls down, and they walk up to it saying "do you think it's dead?" just before it jumps up and kills them. Compare that to the Predator "mini-gun scene" where they see the monster and fire pairs of guns, grenades ... for 30 seconds blowing up trees until the mini-gun is spinning empty with smoke pouring out and we see they've created a new clearing in the forest. That's how you do it! And they didn't even kill it. There is no such thing as overkill!
The second most-(mis)quoted line from Aliens (after "Game over man!") is probably "nuke it from orbit -- it's the only way to be sure". They're screwing around with flamers trying to do minimal damage to an expensive converter plant and Ripley jumps to nukes, which seems like massive overkill, but she was right.
"The Forever War" isn't as well known, but won awards when it came out in 1974. In a scene the space marines are attacking an alien base and getting their instructions from the Sergeant about a building: "There's only one of them though, so we don't damage it any more than we have to. Which means ... we blast it to splinters if I think it's dangerous". That's funny and rings true.
It's a go-to joke. This week's Supergirl has a character discuss how to kill Lex Luther again. He says afterwards they can burn him and spread the ashes on planets with inhospitable atmospheres in different galaxies (because when killing supervillains there is no such thing as overkill).