The Comedian, although that had nothing to do with the (primary) motivation behind his murder. By the end, some readers feel some sympathy for him. But he's still an asshole.
Moloch, though he hadn't been an asshole in years.
Gerald Grice. The plot needed to show Rorschach violently murdering someone, to establish the full onset of his insanity. Grice being a murderer himself makes the story a very solid Black and Grey Morality type.
As bad as Doctor Doom is, King Vladimir Vassily Gonereo Tristian Mangegi Fortunov, the former ruler of Latveria, whom Doom deposed and killed, was a far worse tyrant by most accounts. The same goes for anybody who successfully overthrows him; every time it happens and he later seizes back his power, the citizens are always glad to have him back.
The Punisher usually goes after some rather nasty people. The people he kills tend to be even worse. At least one (if not two) exception was the Punisher being unwillingly wacked out of his skull on drugs.
Most people who get beat up by The Hulk usually have it coming. Bruce Banner just can't stop running into assholes who go out of their way to make the poor guy mad.
Most of Johnny's victims are this. Or are implied to be people like this. Or hung out with people like this. Or stood too close to people like this (i.e., around two kilometers). Come to think of it, Johnny doesn't really discriminate once he's gotten going, but it takes a soon-to-be Asshole Victim to trigger his homicidal rampages... Mostly. Most of the people he takes back to his Torture Cellar (who will be dead pretty soon) are prime examples however. Or at least implied to be.
Jimmy, Johnny's creepy fanboy/admirer, murders several innocent people for no reason other than being like his "hero", Johnny. But without a doubt his worst crime is raping and killing an innocent girl, just because she looks like another who dissed him in school. He's the title character with no sympathetic qualities. When the reader witnesses his brutal murder at Johnny's hands, not a single tear is shed.
Tommy Monaghan, the titular protagonist of Hitman, only takes contracts out on those he considers to be "bad" people.
In the backstory of Kingdom Come, Magog kills the Joker while he's in police custody. This is The Joker, and he was arrested because he went on a rampage in the Daily Planet offices and killed 75 people — including Lois Lane. When Superman protests, the public sides with Magog for this very reason.
This kind of mindset is gradually deconstructed, however. The public deciding that it's okay for heroes to kill someone just because people think they're an asshole eventually leads to an entire generation of out-of-control and reckless heroes who don't hold back for anything — including innocent bystanders.
Speaking of the Joker, he kills Sheila Haywood, Jason Todd's mother in A Death in the Family. She'd lost her medical license for performing back-alley abortions (one of which killed a patient) and was embezzling from her aid agency. She turned over her son to the Joker to save herself (instead of using her gun), who decided to kill both of them anyway.
In the first Deadshot miniseries, Deadshot—at the time a Boxed Crookworking for the government—went on an unsanctioned spree of torture, maiming, and murder. However, since every one of his victims was complicit in the kidnapping, rape, and murder of Deadshot's young son, neither his boss nor the readers cared all that much.
Anyone the Secret Six killed in their eponymous series for the exact same reason as the Punisher example above.
As he's moved to being a more heroic and less serious character, Deadpool often ends up with these kind of targets when he's working as an assassin.
Much like for the Punisher, the people Spawn kills usually are douches, making it easier for the fans to cheer as he dismantle them gruesomely.
When X-23 kills Zander Rice, she puts away the claws and beats him for ten minutes in a truly brutal No-Holds-Barred Beatdown. Rice tortured and both physically and emotionally abused Laura all her life to that point,note Forcibly activating her Healing Factor at age seven with fatal dosages of radiation, removing her claws one at a time for coating in adamantium without anasthesia, torturing her into responding to an olfactory trigger that sends her into a berserker rage whenever she smells it and using it to force her to murder the only two people at the Facility who cared about her, intentionally leaving her behind to be killed on a mission out of spite, and placing her in the "care" of the sadistic Kimura, among other things. And he only did this because Wolverine killed his father and Laura was created using his genetic material. so it makes the scene far more satisfying to read than watching someone get pounded into an unrecognizable pulp would normally be.
Happens in quite a few EC Comics, as often someone invokes a Karmic Death by driving another person to kill them.