It's worth keeping in mind Tertullian's description about how Marcion came to his views: Against Marcion, Book 1, Chapter 1-2:
http://www.earlychristianwritings.com/t ... an121.html
For the cynic Diogenes used to go about, lantern in hand, at mid-day to find a man; whereas Marcion has quenched the light of his faith, and so lost the God whom he had found. His disciples will not deny that his first faith he held along with ourselves; a letter of his own proves this; so that for the future a heretic may from his case be designated as one who, forsaking that which was prior, afterwards chose out for himself that which was not in times past. For in as far as what was delivered in times past and from the beginning will be held as truth, in so far will that be accounted heresy which is brought in later. But another brief treatise will maintain this position against heretics, who ought to be refuted even without a consideration of their doctrines, on the ground that they are heretical by reason of the novelty of their opinions. Now, so far as any controversy is to be admitted, I will for the time (lest our compendious principle of novelty, being called in on all occasions to our aid, should be imputed to want of confidence) begin with setting forth our adversary's rule of belief, that it may escape no one what our main contention is to be.
Chapter 2
The heretic of Pontus introduces two Gods, like the twin Symplegades of his own shipwreck: One whom it was impossible to deny, i.e. our Creator; and one whom he will never be able to prove, i.e. his own god. The unhappy man gained the first idea of his conceit from the simple passage of our Lord's saying, which has reference to human beings and not divine ones, wherein He disposes of those examples of a good tree and a corrupt one; how that "the good tree bringeth not forth corrupt fruit, neither the corrupt tree good fruit." Which means, that an honest mind and good faith cannot produce evil deeds, any more than an evil disposition can produce good deeds. Now (like many other persons now-a-days, especially those who have an heretical proclivity), while morbidly brooding over the question of the origin of evil, his perception became blunted by the very irregularity of his researches; and when he found the Creator declaring, "I am He that createth evil," inasmuch as he had already concluded from other arguments, which are satisfactory to every perverted mind, that God is the author of evil, so he now applied to the Creator the figure of the corrupt tree bringing forth evil fruit, that is, moral evil, and then presumed that there ought to be another god, after the analogy of the good tree producing its good fruit. Accordingly, finding in Christ a different disposition, as it were--one of a simple and pure benevolence--differing from the Creator, he readily argued that in his Christ had been revealed a new and strange divinity; and then with a little leaven he leavened the whole lump of the faith, flavouring it with the acidity of his own heresy.
"
His disciples will not deny that his first faith he held along with ourselves; a letter of his own proves this'. This suggests that Marcion did indeed start out with views that were consistent with proto-orthodoxy, and only later developed his docetic views. "
I am He that createth evil" gets quoted a lot by atheists nowadays, so it is plausible that Marcion had problems with that statement and so came to his own readings of Christian texts.
There is also the question of Cerdo/Cerdon, whom Tertullian and others mention a few times as either the co-heretic of Marcion or even Marcion's source. So Marcion may have been part of a school of thought.