Jason Aaron is not the man to bring forth the hidden facets of Thanos' character. When presented with such a rich and conceptually heavy figure, he gives us the standard narrative of a sociopathic youth. Let's back up and think about the fact that not merely is Thanos a rich character, but his milieu is even more interesting: he is a mutant born in a society of gods, the son of the ruler of the Titanian Eternals. (Thanos and his race of Titans were created in 1973, three years before Kirby introduced the Eternals to Marvel, and later retconned into being an offshoot of the Earth Eternals.) Thanos hails from a race of supermen who have built a technological Utopia in a moon of Saturn. The best Aaron can imagine to do with this setting and this backstory is to give us the story of Lil' Thanos as a burgeoning serial killer. I don't necessarily blame Aaron: he is a competent writer who is resolutely hamstrung by a severe lack of imagination. He doesn't strike me as someone who has done much in the way of serious reading, because all of his storytelling touchstones appear to be other comics or movies or popular fiction. This actually works to his advantage in telling the stories of a character like Wolverine, because Wolverine is himself such a pastiche of received modes of hard-boiled mens's adventure, action, and noir storytelling that his best stories usually function themselves on the level of high pastiche. But Thanos isn't Wolverine, and writing an overly-literal interpretation of Thanos' childhood and development is one of the most fantastic examples of a creator completely missing the point that I have ever seen. I don't doubt that Aaron knows a fair bit about criminals and sociopaths, but that is hardly to the story's favor. We don't need to read a psychological thriller about a young murderer-in-training, we need to see gods and monsters whose every thought and deed is dripping with metaphor.
That's what cosmic is all about. Cosmic isn't about telling crime stories or action stories or thrillers in an exotic setting. Cosmic is about heightened reality, a form of storytelling defined by the absence of familiar referents, riven with symbolism, and steeped in fanciful mythology. Kirby got that, and he helped create the very idea of cosmic storytelling in comics because he understood that one of the best ways to tell "real" stories in childrens' comics was to put those stories into outer space and other worlds, and thereby to make them about everything that they couldn't be about if the stories had been stuck on the planet Earth. Starlin understood this too - in fact, he devoted much of his career to developing fantasy in comics as a springboard with which to talk about all the weird stuff in his own psyche. Jason Aaron isn't much of a fantasist. Thanos Rising is a story about Thanos, yes, but it's not a very good one: it's a very mundane story about a young killer such as you have probably seen and read many times before. Aaron would probably point to its familiarity as a feature, with the observation that the ways in which sociopaths grow up are often very similar, and that the most truly unsettling facet of these narratives are the ways in which the characters transform under the influence of their banal context to become monsters. (See Derf Backderf's My Friend Dahmer for a perfect distillation of this principle in action.)
This is all well and good, and certainly, the outlines of Thanos' story were all put there by Starlin. But Starlin knew better than to dwell on the sordid details of Thanos' upbringing: he wasn't a real character, after all, he was a metaphor. I know I said above that Thanos was a more interesting character than Darkseid - well, guess what, I lied. Neither of them are real "characters." They're lines on paper. They're symbols. Darkseid is a rich metaphor, as is Thanos. I think Thanos is a tad richer for the simple reason that, because the baseline of his character is so steeped in adolescent angst, he can always be placed in the position of needing to grow up in some manner - as we saw throughout the 90s, after the Infinity Gauntlet, when Thanos was left to his own devices without an overriding need to dominate the universe, and became almost a kind of pilgrim. Keith Giffen wrote perhaps the best non-Starlin Thanos when he picked up on this aspect of Thanos' behavior following Starlin's own short******d return to the character in the early 00s. The point is, there are so many interesting things that you can do with this guy, and telling the story of how he was born and became a child serial killer is probably the least interesting of all possible options.