I think we need to be careful projecting things like religion onto non-human intelligences. AFAWK, religion is one of the most uniquely human practices/rituals that we exhibit. There is literally no analogue for it in the natural world. If, as I'm sure most of you will except, we as a species are the sum of a sequence of extremely unique, nigh unrepeatable events and circumstances, the notion that something like religion would somehow inevitably develop in an alien intelligence is probably way too far of an intuitive leap. Indeed, being religious fanatics would hardly make them seem 'less' human in their motives.
I do agree that it would be an intriguing reason/excuse/justification for an invasion (certainly among the more plausible of those thus far presented), but it would still seem like allegory rather than something truly alien. The problem, then, is that we as humans just can't think "alien" enough to make something truly original as far as invasions go. In other words, "we" only invade for these reasons. Ergo, aliens must invade for the same reasons.
I suggest a thought experiment that may truly yield alien results. Come up with a reason why a race like, say, the Vulcans or the Asgard, rather than Klingons or Goa'uld, would invade (destructively) our Earth (c.2011).