Barret, along with Biggs, Wedge, and Jessie are in favor of ecoterrorism, for they are the most supportive of bombing Mako reactors. They all fulfilled roles in the destruction of both the Sector 1 and Sector 5 Mako reactors. Barret seems to regret this much later in the game, and by this point Biggs, Wedge, and Jessie have died fighting Shinra troops. However, he still fights for the planet, and is fine with the assassination of the Shinra leaders Scarlet, Heidigger, and Hojo, along with some Shinra Soldier 1st class random battle enemies that can be encountered in that area. Barret is good at rallying support for the planet (he founded AVALANCHE), although later on in the game he tends to get somewhat confused, and not understand the Lifestream that well as information gets more complicated.
(During the bombing of the Sector 1 Reactor)
Barret: The planet's full of Mako energy. People here use it every day. It's the life blood of the planet. But Shinra keeps suckin' the blood out with these machines... The planet's dyin', Cloud!
The previous quote was to get support from Cloud at the beginning of the game when Cloud only cared about his paycheck as a mercenary.
(At Fort Condor)
Barret: I'll do it! I'll protect them [the condors] from Shinra!
This quote shows that Barret cares for condors, who are in danger of extinction.
(Before fighting Sephiroth in the Planet Core)
Barret: Not only Aeris... Holy is the Prayer of AVALANCHE... of Marlene and Dyne... And everyone on the planet!
This last quote shows that Barret has changed his mind about not fighting for the planet, in regards to Cloud's "Reasons for Fighting" speech. Even though he no longer supports bombing reactors, he still wants to protect the planet and everyone on it, and he mentions it as a reason for fighting Sephiroth.
