Anyone who idolizes Castle either fundamentally misunderstands the character or is a psychopath.
He's an anti-hero by definition; when written well (the definitive run is Ennis' Punisher MAX work), his lifestyle is depicted as being the sad, empty, fugitive existence that it is. The death of his family caused a schism in his heart from which he could not recover, and the past several decades have been him arrested in the "anger" stage of the Kubler-Ross grieving model.
He commits mass murder on those whom he deems evil (and to be fair, they almost invariably ARE evil) as much as an act of indirect retribution for the murder of his family, as the fact that he knows of no other way to exorcise the despair of their loss. People misconstrue Castle as being a man bent on vigilante justice, using his skills as an operator honed in Vietnam to facilitate a one-man crusade, which is only superficially true.
But in fact he's an emotionally arrested man insane with a grief he cannot let go, mourning in the only way he knows how.
The general consensus is that he's the best to ever write Castle, by a wide margin.
He did a Marvel Knights Punisher run with Steve Dillon ^^(omni) (i.e. the creative team behind Preacher).
But the run that cemented Ennis' reputation as the end-all-be-all Castle writer (it's Garth Ennis, after all; of course we can expect great things from him) was his Punisher MAX run ^^(omni ^^vol ^^1 ^^| ^^omni ^^vol ^^2) on Marvel's adults-only imprint MAX. To my mind, it's not only the best Punisher material bar none, but one of the all-time greatest Marvel stories.
Among the work he did there was Born, about the genesis of who Castle would ultimately become, catalyzed by his experiences in the Vietnam War. Larosa's depiction of Castle as a grizzled, profusely scarred, seething hulk of a man, rather than the untouched, slick-haired, movie-star-handsome depiction he usually receives, is one of the definitive visual interpretations of Castle AFAIC.
Here's a collage of the nine artists who worked on Punisher MAX, of Frank at various times of his life, to give you some idea of where he "goes."
The Ennis MAX material is available in more affordable TPBs as Punisher Max Complete Collection vols 1 - 4. Also, the followup prequel with Parlov Punisher: The Platoon is must-read as well.