If he's into the "defend nature" thing, his trauma could stem from seeing huge destruction of wildlife that he's powerless to stop. This might lead to a "ends justify the means" mindset: defend nature by destroying its despoilers. As a Blackguard, he'd try and stop exploitation of natural resources, pollution etc., by going and outright killing the offenders - even if it's just Luke the Logger, his wife and his children.
Mechanics: Rangers come in many variants - have you considered
Wildshape Ranger and Mystic Ranger (some Dragon issue)? Both provide a nice upgrade over just regular Ranger, and some interesting abilities. A Wildshape Ranger qualifies for Master of Many Forms at 6th, and a few levels of that can really bring it home where melee power and general flexibility are concerned. MoMF requires Alertness - consider giving the LG Ranger a
Celestial Bloodline to provide the feat for free, as well as some other goodies. This just adds to the drama of falling to become a Blackguard later.
Mystic Ranger, OTOH, can qualify for any number of arcane PrCs, including the very good Abjurant Champion, and is generally considered quite powerful.
Another thing you want to keep in mind is that Clerics and Artificers are very good at buffing. With those two guys to back him up, even a simple boring single-class standard Ranger can mix it up - so make sure those two don't play the egoist game.
EDIT: Also, w00t for seven-seven-seven posts, and level seven!