Humanizing a character isn't necessarily a problem. But even if he was a human character this story would still be plain bad. The real problem is that his character is not being used in any sort of dramatic or interesting way. He isn't driving the story, and there isn't much of a story anyway.
He's running around killing lizards and cutting a few people up for no real reason. He's apparently spending months locked in a battle of wits with the local rock apes just so he can cut a live one up. He runs around siring kids because he has no clue what else to do, then runs around and kills them because someone tells him to.
This far in there should be something more significant going on. So far he just seems to be going through the motions.
He's not descended form past humans. As mentioned before, the Eternals/Deviants are offshoots from an ancient species of hominids from which humanity also eventually evolved.
Thanos would be as much partly human as Oliver the mutant chimpanzee is.
Thanos is one of those villains I want to be like hanibal. I don't want to understand him or relate to him. I want him to be this force of evil and malice that has been hiding under your nose for ages who isn't relatable at all. Which is why I didn't like how they did a movie on Hannibal explaining why he is the way he is. When someone reaches a certain level of villainy, some details are better left to the imagination, the odds of being disappointed when you see them played out are going to be high regardless. Like what if they came out with an origin story on the Joker? Exposing every mystery of his past? IMO he wouldn't be half as terrifying as he is.
