There are some opponents you want to defeat. There are some opponents you want to kill. Then there are the opponents whom you're not satisfied with until you've stabbed them in the back, shot them a lot in the head, blown them up, unleashed the hounds on them, electrocuted them, jumped up and down on them, and then shot the remains into space on a rocket full of time bombs heading towards the sun.
This trope comes in three categories (with some overlap): the killer either loves violence for its own sake, or wishes to express dislike for the target, or (usually due to fighting The Juggernaut or an Implacable Man) knows that It's The Only Way To Be Sure. There Is No Kill Like Overkill may be played for comedy or for drama. If the former, another character may remark "I think he's dead already" near the end of the process.
May be employed successfully: see No One Could Survive That!. Unfortunately often leads to Never Found the Body, thus setting up Only Mostly Dead or Unexplained Recovery. Joker Immunity means even overkill is useless, but even that is better than a No-Sell.
It is sometimes justified if the opponent can come back From a Single Cell, or if resurrection is possible in the setting but requires the body to be mostly intact (in which case enough overkill to demolish the corpse serves the useful function of rendering the target Deader than Dead).
Much, much more common in fiction (especially of the 'revenge fantasy' type) than in Real Life violence. To a strategist, overkill is Awesome, but Impractical because the risk of enemy survival is vastly outweighed by the certainty that you're spending a fortune in time, material, and (usually) manpower to ensure the enemy's death. The weaker and less numerous the target, the more wasteful overkill is.
For common ways to overkill, see Beam Spam, Wave-Motion Gun, Megaton Punch, Gatling Good, Boom, Headshot!, Your Head Asplode, Grenade Spam, Multiple Gunshot Death, Macross Missile Massacre, No-Holds-Barred Beatdown, Stuff Blowing Up, Home-Run Hitter, More Dakka, Multi-Directional Barrage, BFS, BFG, Rapid-Fire Fisticuffs, Pummeling the Corpse, and/or Nuke 'em. Said methods are almost always more overkill with a Critical Hit.
For situations in which the overkill destroys the whole point of the combat, see Pyrrhic Victory.
Compare Chunky Salsa Rule, Critical Existence Failure, Deader than Dead, Disproportionate Retribution, Rasputinian Death, Refuge in Audacity, Surplus Damage Bonus, and Video Game Cruelty Potential.
Contrast Cherry Tapping and Once is Not Enough. Characters who don't have enough overkill at their disposal just might resort to crossing the Godzilla Threshold instead. If it's an animal, it's often a Black Comedy Pet Death.
This is occasionally related to a major character's death, so beware of unmarked spoilers!
