These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Law Abiding Citizen
Complete Monster: Clarence Darby rapes Clyde Shelton's wife, murders her, savagely beats Clyde with a baseball bat, and then murders Clyde's young daughter. They don't even show the audience that scene; it's so horrible. He just takes her into the next room. The line "Don't worry...kids like me." could hint that he raped the girl too. He also attempts to murder Clyde. He proceeds to lie, saying that his partner did it (when, in fact, the partner had tried to talk him out of it), and subsequently laughed his head off when he hears about his partner's agonizing death.
Darkness-Induced Audience Apathy: This feeling hits home for viewers who become increasingly disillusioned with Clyde's actions halfway through the film. Clyde starts off as sympathetic, but he eventually becomes worse than the madman (Darby) who sparked off his rage in the first place. As for the attorneys and lawyers on the other side, they're immoral strawmen made to look as corrupt as possible, except for Cindy, who dies thanks to Clyde. When one realizes Clyde's hypocrisy reaches the level of Jigsaw it's impossible to root for anybody. See Black and Gray Morality.
Draco in Leather Pants: There are quite a few people who present Clyde as a hero fighting a corrupt system, ignoring that many of the people who die at Clyde's hands were simply employees doing their jobs and not actively trying to wrong him. That and he killed his cellmate simply to advance his plot. Clyde might be sympathetic, but the crimes he committed are still wrong.
Faux Symbolism: The fires that surround Clyde the last moment we see him in the film before his death.
When Sarah and the others are blown up outside the prison, Clyde is shown in priest-like black clothes and white undershirt, fingering his daughter's bracelet which resembles rosary beads.
Hilarious in Hindsight: At one point Bray says "spies are a dime a dozen", which was included in the trailer. This unintentionally mirrored a line from Team Fortress 2's trailer "Meet The Spy", which has the Scout saying "[Spies are] dime-a-dozen back-stabbing scumbags!" The payoff? Well, see Law Abiding Engineer.
Magnificent Bastard: Clyde Shelton. You may not approve of the idea that guides him, but you have to admit and admire his style and execution.
Initially, we can sympathize with Clyde Shelton's desire to kill Clarence Darby as vengeance for killing his wife and kids. But then there's how he goes about it: paralyzing his body, taking him to his lab and chopping of his limbs with a chainsaw, making a video of this, and sending the video to Nick Rice.
He definitely crosses it when he murders his cellmate for no other reason then to advance The Plan. Although there are many different views, this is the most widely cited.