Played straight when the subject Helen has been experimenting on for the entire run of the comic dumps her even though she has fallen in love with him, and takes over another secret lair, which in another timeline leads to Helen's near-death and existence as nothing but a brain in a tank.
Xykon sends several goblins to their deaths simply to observe the effects of walking into Dorukon's Gate. They invariably explode, but Xykon is a For the Evulz kind of guy, and thinks it's funny. During the fight with the PCs he shatters Roy's ancestral sword, and Roy picks him up in a bout of Unstoppable Rage and throws him into the gate himself. Of course, it takes more than the destruction of his body to permanently destroy a lich, but it does set his plans back months while he seeks out a new Gate.
Tsukiko is a necromancer with a rather disturbing fondness for her undead creations, believing that, since the undead are the antithesis of life and Humans Are Bastards, the undead must be misunderstood, lovable souls. Redcloak demonstrates his own belief that the undead are nothing more than tools to suit a purpose by using a 'Command Undead' spell to force Tsukiko's thralls to devour her and each other.
Tarquin wants Elan to be in command and in action, but his forces refuse to fight Tarquin's son, thus forcing Elan into a support role; once Haley points out that it subverts his father's plan, Elan is delighted with it.
Elan: (singing) Rub, rub, rub, rub my father's face in my uselessness!
In Start of Darkness, while they may be the arguable good guys, the massacre of Redcloak's village was arguably what led to the conquest of Azure City and the near destruction of the Sapphire Guard.
Lord Shojo willfully deceives the Sapphire Guard by using the Order of the Stick to investigate the other gates (Soon's Oath forbidding the paladins themselves from doing it), and fakes his senility to avoid being assassinated. While discussing his ruse with Roy and Belkar, Hinjo and Miko arrived. Word of the Giant says that Shojo's deceptions, while portrayed as a moral action, ended up causing Miko to flip out and kill him.
Girly: Called out by Clone Sidekick Hammergirl, as she is smashed to death with her falling hammer to reveal a hollow center!
In Juathuur, the juathuur deserters with whom Meidar is fighting left the path in large part because the Secret Test of Character she had for each of them left them disillusioned. One particular former juathuur, Rowasu, kills her in the end.
Nature of Nature's Art: SV, why did you decide that harnessing the power of malice would be a good idea? Sure, it's a rather unorthodox definition of malice, but that doesn't change the fact that it ultimately killed you - or "[devoured] you as you [devoured] it", as you put it. At least you realised it at all.
Yuri goes full cyborg, incorporating more and more weapons into her body. Martina decides that Yuri is going too far and needs to be restrained. To incapacitate Yuri, she uses a bomb that Yuri herself had commissioned from the Eebs.
The Eebs created the brain clamps that would eventually be used to cripple their psychic powers and rob them of free will. In fact, they were the ones who started brain-clamping each other in the first place; when outsiders decided to enslave them, all that was necessary was to kill the half-dozen of them who remained unclamped.
Equius's bodybuilding obsession leaves him strong enough that a rifle falls apart in his handsnote Top left corner of the room at the top of the stairs, after "hiding" as Nepeta (the catgirl; talk to her to take control).... a few strips later, Gamzee wounds him with a bow and arrow, then walks slowly across the room to garrote him.
Vriska stabs Tavros with his own lance after he tries to kill her, killing him.
As the Thief of Light, Vriska has apparently complete ownership over fortune, with the ability to alter luck-based events to her whim. When she refuses to back down from going off to fight Jack, Terezi makes her a bet her ego could not refuse - a simple coin flip. Heads, she stays. Scratched heads, she goes. The double entendre of the word was intentional and understood by both to be death. Vriska forces the coin to yield the scratched head, gambling that Terezi did not have the courage to go through with it. She was wrong in the Alpha Timeline, and dies in accordance with their agreement.
In Mitadake Saga, Kira has their own name written into his Death Note by the very girl he betrayed and left for dead - in her own blood. This is realized far too late.
Subverted here: Col. Tora Scobee, a bionic wolf, plants a bomb in the office of his rival, Borzoi, who is a Russian Wolfhound. Borzoi sees the bomb just as Scobee is getting into his car, and Borzoi chucks the bomb at the exact moment Scobee is inside. The last scene with Scobee in a full-body cast and his boss, Stinky, berating him.
Stinky: Scobee, this rivalry with Borzoi has got to stop. We're running out of spare parts!
Petey: You must demonstrate the ability to defend the surface of this planet. A Frigate armed with light shields and a fully operational plasma lance has just entered orbit. All you need to do is shoot it down. Petey: In ten minutes it will be overhead, at which point it will destroy this building and any who remain inside.
U.N.S. requests the extradition of mercenaries who blew up a TV company, pulled an insider job in process and planted fake evidence. After the local government itself hired them to attack the company via special interest group as a sting operation and part of a social engineering program, also made money on this insider information, and planted fake evidence. The case is clear, so he's most eager to cooperate...
Petey: but I expect that you will get quite a rise out of them when you explain to them that these proceedings will be made public as part of the extradition process.
...and since they didn't learn fast enough to drop it fully: