So...obviously everyone knows Emerald Twilight was redone at the last minute to the story we ultimately got. Obviously Hal was never supposed to go insane and wipe out the GL Corps in an attempt to resurrect Coast City.
However, Ron Marz did attempt to explain Hal's sudden and drastic turn from issue #47 to #48 in that Hal was clearly in denial about the destruction of Coast City.
Weak explanation perhaps, but take it for what you will.
Starlin wrote loads of Thanos rip offs, like the High Papal from Dreadstar