Originally posted by darthgoober
The soul gem can't steal the souls of abstract types, Warlock specifically noted that the souls of beings like Galactus are beyond it's ability to manipulate. What's more, soul absorption is a tactic that Warlock rarely ever used because it's inherently perilous. He was always worried that a soul he absorbed might actually overwelm his own... that's why he always avoided using the tactic on Thanos.
He stole part of the soul of Star Thief, an abstract level being.