The best villains all seem to have rather rough upbringings: Voldemort was abandoned by his mother and unwanted by his father; Darth Vader was a slave on a hot desert planet in Star Wars and the Joker himself describes his father as being ‘a drunk and a fiend.'
And Thanos is no different. Born as one of the last sons of colonists Mentor and Sui-San on Titan, his introduction to life was harsh. He was born grey and misshapen in comparison to others such as his brother, Starfox. As a result, his parents shunned him and never gave him the same loving treatment as his siblings.
And Thanos got his revenge by building an army. Once done, he returned to Titan and declared war, murdering his mother Sui-San along the way. How about that for acting out?