Schlock Mercenary has its fair share of this, frequently killing off supporting cast members. Although anything short of a headshot can be healed thanks to Applied Phlebetonium, and major characters were brought back through Time Travel.
Being the demolitions tech for the Toughs is pretty much a one-way ticket out of the strip...
To wit: The strip has killed, since the beginning, six major characters just among the Toughs, including two who could easily be billed as main characters, and so many supporting Toughs it's hard to count. And these are just the ones who HAVEN'T come back.
To be fair, however, that's spread out over twelve years. You can certainly start reading any given arc and assume that no Toughs will have died by the end of it (not even any redshirts).
The pre-It's Walky Roomies featured the (then) shocking death of Ruth. Her death marked a Cerebus Syndrome moment in the strip's history (The strip started the transition to It's Walky in the immediate aftermath) and served to show that the gang's wacky hijinks were no longer consequence-free. It also set the "No warning" tone for many of the deaths to come (Dina's in particular).
Yet Another Fantasy Gamer Comic has main characters die quite often, and with little drama. It helps that there are quite a few main characters in each arc. Only one of them, Glon, has ever been brought back.
Although set in a world with functional resurrection magic, The Order of the Stick has featured a number of shocking deaths. It uses the interesting loophole in all D&D resurrection spells: the dead person's spirit has to be willing to return.
Lord Shojo, and Therkla would, for various reasons, rather stay dead than face their lives again.
Miko Miyazaki wasn't exactly well-liked, and was cut in half.
Then Durkon Thundershield bit it, though he got better...sort of.
Strip #913 sees the death of major antagonist and Elan's evil twin, Nale, with his body being zapped to dust and scattered to the winds.
A Game Of Roleplay attempts to set a record for major character deaths with four unexpected deaths by the end of the first chapter.
These include one of only three players charactor (twice), the air to the throne (another PC) and the quest giver for the main campaign.
This is even more impressive when you consider that the strip is based on Game of Thrones and so follows an already set out plot. This is of course changed to suit the author's new plot.
Despite being a fancomic and using characters from TV Shows, Powerpuff Girls Doujinshi doesn't skimp on this. Deedee is shown killed during a flashback, taking a laser blast for Dexter. Mandark, in his attempt to kill Dexter, lets himself be blown up during his base self detonation. Blossom is temporally drowned while being held captive but brought back to life. During the side story, Atomic Betty's crew (Sparky and X-5) are killed after stumbling upon one of the big bad's bases. Betty nearly killed herself if not for Amazo finding her.
Wrongside: Beginning has not been hesitant to kill characters off. And has even parodied it in a joke strip.
Goblins. Many characters die as soon as you start getting attached to them. The tagline for Book Four's climax was "EVEN MAIN CHARACTERS CAN'T LIVE FOREVER."
Hitmen For Destiny killed off a character who was explained to be extremely important quite early on. Recently another major character was unexpectedly killed off. Both of these characters were very popular among the fanbase. It looks like nobody is safe at this point.
Juathuur has lot of death in it, and no way shown to resurrect people. The trope is established with Bivv's death and comes into full force with the Battle of Erab Adur.
Sluggy Freelance. Monica's death was especially shocking for its sheer randomness, stabbed in the back by Oasis as she arrived; ZoŽ's death, and the weeks spent convincing the audience it was real, was a shock, but at least that was in an action scene. At one point in the strip's early days, the entire Cast Herd of ZoŽ's college friends were killed off in a horror film parody, and later, the sequel killed off one of the main characters' housemates during a victim montage.
Homestuck as of January 2011. All deaths prior to this were throwaways; Dave's doomed timeline duplicates, dreamselves, real selves that are just replaced by dream selves (like Sollux, but he had two so it didn't matter), John, Vriska, and Aradia's ascensions to God Tier, random mooks and carapaces, and probably more. Then Eridan flipped and killed Kanaya and Feferi. Vriska killed Tavros. Gamzee killed Equius and Nepeta. Kanaya Came Back Strong as a Rainbow Drinker and sawed Eridan in half. As per more recent developments, Vriska is dead, either having received a Just death or losing God Tier immortality due to Spade Slick breaking the God Tier clocknote The exact case was intentionally left vague. Hussie sure loves us!
Luckily, dead characters can still show up in dreambubbles. Unluckily, Lord English is destroying dream bubbles, which permanently destroys any dead characters in them at the time...something which was first demonstrated in a flash in which an entire planet of dead characters was instantly destroyed.
As an example of how anyone can die. The first thing Lord English did before he went on his dream bubble rampage is kill the author.
Game Destroyers has killed off or otherwise incapacitated a number of main characters to date, namely Bojangles, Ace, Cedris, and the Nintendo Otaku. Only a small handful of main characters will likely never be considered when it's time to kill someone off.
Counting Not just anyone but everyone in the town of Thirston. Including a bird in a bird bath.
Something Positive. The author, Randy Milholland, has made it quite clear that nobody - beloved or despised - is immune from the Grim Reaper. The responses when he invokes this trope tend to vary; in one case, someone told Randy to his face that the death of a certain character didn't actually happen. And if you tell him he can't, for whatever reason, kill off a character, Randy will kill said character anyway out of spite, even if they weren't supposed to die.