“We were considering showing (Thanos’) backstory, but ultimately felt it wasn’t necessary for the movie… (Infinity War is still) told from his (Thanos’) point of view.”
For now, you can check out what the Russo Brothers had to say about cutting out Thanos’ backstory over at Total Film.
As for what Thanos will be like in Infinity War and the reasoning behind his motivations, Josh Brolin recently shed some light on the subject.
After the last trailer for Infinity War was released in March, many reports from the set were released with interviews conducted between the Russo Brothers over the summer.