It seems as if every member of the cast and crew of this film set out to make Harry Brown as despicable an experience as possible. The characters are reprehensible, the colors and lighting are sickly, the dialogue is profane, the violence is-- technical term here-- squicky. Michael Caine goes Charles Bronson on a bunch of teenaged punks and druggies terrorizing his rundown English neighborhood. His wife and child are dead; his best friend is killed. He is a man with nothing left to lose.
The film has a lot in common with Garth Ennis' Punisher MAX comic book run, I would say; an old man accepts violence as an answer to violence, and sets about punishing the guilty. The filmmakers ensure that the audience thoroughly despises Caine's opponents, until we at home are actively supporting the elderly explosion of murder and mayhem that occurs in the back half of the movie.
Many will compare this to Clint Eastwood's Gran Torino, and while the trappings are the same, the films are tonal opposites. At its heart, Gran Torino was an anti-violence film, a cathartic purging of Eastwood's Dirty Harry mentality. Harry Brown remains dirty, advocating bloodshed in response to bloodshed, glorifying vigilante violence, and making the audience complicit therein. But hey, at least Michael Caine can walk through that underpass again.