I Didn't Mean to Kill Him

An alternative to the Motive Rant has the perp, when cornered, explain how the murder was all an accident. Their excuse can be almost anything, but the most common explanations are

"It was just a joke!", when a prank or practical joke goes horribly wrong.
They were having an argument with the victim and accidentally shoved/nudged/chased them over the precipice or down the stairs.
"I didn't mean to hit him that hard!": They did mean to hit him, just a teensy bit, or some such thing.
"I only meant to make him sick...", usually so that the victim would be unable to do something and the killer could step into the victim's place. It may be a competition, a presentation, or a trip. Or almost anything else.

After making the I Didn't Mean to Kill Him declaration, they'll often go on to explain that they dismembered the corpse and hid the pieces in a septic tank because they "panicked". In such a spiel, they will always, every time, utter the line "I didn't mean to kill him/her", as if that's supposed to make it all better.

Examples of I Didn't Mean to Kill Him include:

Comic Books

During Infinite Crisis, Super-boy Prime after killing Baby Wildebeest and Pantha
- And Bushido

Film

Subverted in Legally Blonde where Chutney insisted that she didn't mean to kill her father, but meant to kill Brooke instead.
Seymour of the original The Little Shop of Horrors says something along these lines. He even says this from inside the plant after his face appears on one of the flowers. His mother and Audrey, of course, are shocked.
Dorothy in the movie version of The Wizard of Oz says this about both Wicked Witches. In the book, while she may not have known water would kill the Witch of the West, she meant to throw it on her.

Literature

In the Dragonlance book Dragons of Spring Dawning, we find out that Berem rarely talks or even interacts with the outside world because in his mind, he's reliving the day that he accidentally killed his sister, Jasla. He was prying a gemstone out of a ruined temple, and Jasla told him to stop. She grabbed him and in a rage, he shoved her, causing her to fall and get impaled on a stone fragment. Turns out that the death of Jasla is what allowed Takhisis to come back to Krynn and set in motion the War of the Lance.
A subversion of this occurs toward the end of Harry Potter and the Deathly Hallows, when Harry tries to explain to the ghosts of Remus, Sirius, and his parents, that he never meant for any of them to die. Sniffle.
- Also, in the sixth book, when Harry inadvertantly slashes Draco to bits with the Sectumsempra spell (nearly killing him), he gasps, "No- I didn't-"
Troubled but Cute Johnny in The Outsiders says this when he he kills Bob, the Soc who was drowning Ponyboy.

Live-Action TV

Buffy the Vampire Slayer, after accidently killing her mother's boyfriend, Ted. Luckily he's a robot
An episode of CSI, Loco Motives, features a multiple murderer who explains that he didn't mean to kill anyone. He just happens to be prone to causing fatal accidents.
In the revelation sequence from M*A*S*H‍'‍s finale "Goodbye, Farewell, and Amen", Hawkeye sobs, "I didn't mean for her to kill it! I just wanted it to be quiet!", referring to an infant a mother had suffocated intentionally, for the sake of preventing her and the rest of her party from being discovered by the enemy because of the baby's continued crying. Hawkeye had tersely told her at the time, in a moment of desperation, to "Keep that damn thing quiet".
- And to make matters worse, Hawkeye didn't even know it was a baby at first; he was so stressed from staying awake for the last few days without rest that he thought the baby was a chicken that wouldn't stop clucking.

New Media

In Conquering the Horizon when confronted by Hsthressis's mom (who was also the chieftain and high priestess of Hsthressis's people) Evelyn explains that it was an accident and she didn't know at the time that Hsthressis was a person. At first Evelyn didn't fess up to killing Hsthressis and referred to what she did euphemistically (or deceitfully) referring to killing Hsthressis as hurting Hsthressis "very, very badly". Of course when you are literally bringing someone back to life, the distinction between dead and injured might be a bit blurred.

Video Games

Thorvald Two-Stone in Arcanum was sent to the Isle of Despair for killing a gnome with a single punch at a bar. He is completely non-repentant, saying that the gnome had a weak neck. Thorvald did have a strength of 21, which is pretty much the maximum amount in this game.
One sidequest in Dragon Age Origins has you disposing of the "evidence" of deals gone bad, and by "evidence" I mean bodies. The people who killed them all give this excuse.