Thanos' MCU depiction isn't a distillation of a decades old comic character, it's a result of them not adapting the subplot where he wants to bang Death. Seriously, his motivation in the comics is that he has a boner for Death and thinks she'll want to sleep with him if he kills an incredibly large number of people. They hadn't gotten that far into the cosmology by the time they used Thanos in the movies, so they had to come up with some other reason that he might want to kill half the population of the universe.