Since the Russos have already said we won’t see Thanos’ backstory in Infinity War, ideally all this information will be passed along through dialogue/exposition.
Marvel Studios president Kevin Feige briefly touched on Thanos’ backstory when it was confirmed that we would see Titan in Avengers: Infinity War, but Joe Russo provided far more detail when he and his brother, Anthony Russo, were speaking with The Telegraph.
With his people gone, now Thanos has taken it upon himself to enact this same plan on other worlds, which explains why he’s a feared warlord across the galaxy.
Talk about taking ‘the ends justify the means’ to a whole other level.