Constantine is a strange character. He’s an anti-hero, plagued by the ghosts of people he has failed in some way, and capable of quite horrendous things, if a case requires it. He uses magic and the stories revolve around and heaven and their respective worshippers on earth. The stories are very strange, sometimes difficult to understand and sometimes have a political tone. Constantine’s role in the stories is also not decisive. He may watch others solve the case, or he may be the man who gets knocked out upon entered a demon’s lair, waking to find his friends have solved the case. To be frank, I’m surprised that a comic like this became popular. I’m also happy that it did.
You should read Hellblazer if you like horror comics, or if you like strange narratives in general. If you like neither of these things, you should read Hellblazer because Constantine is a great character. He is utterly ruthless and not cruel, he tries to achieve the best outcome, he’s the best someone could hope for in the comic’s world. He will not take the risks that some heroic characters would, because in his world, that risk would actually have consequences, unlike so many stories involving heroes.
You shouldn’t read Hellblazer if you don’t like the art (check a few images online), if you are bothered by left leaning political stories, or if you prefer more traditional narratives and heroes.