THANOS SAVED THE UNIVERSE

"With all six stones I can simply snap my fingers, I call that mercy. The hardest choices require the strongest of wills." ~ Thanos

During Thanos' conversation with Doctor Strange on Titan after he had obtained the soul stone from the planet Vormir, we get to understand a whole lot more why Thanos wants to wipe out half of the universe with the power of the infinity stones. Thanos had faced a great deal of tragedy after the devastating destruction of his home planet and his race - he probably witnessed the slow and painful death of children dying from starvation, people probably having to turn carnivals in other to survive, natural disaster plaguing the globe and worst of all how he was completely unable to do anything about it. All these factors and considering he had foreseen the destruction of his home planet greatly traumatized him, he felt powerless because those in power abolished his warnings and termed him "mad man" and he lived with the guilt that he could've done something, anything to prevent the extinction of his race. This event greatly wounded Thanos and motivated him to seek out the power to bring balance to the universe and prevent similar devastating and cruel form of loss.
"This universe is finite, its resources finite, if life is left unchecked life will cease to exist." ~ Thanos

After many years of invading several planets and murdering half its population Thanos probably realized he couldn't keep up with the vastness of the universe and discovered the best alternative to balancing the universe was obtaining the infinity stones. He figured that instead of having to travel the universe murdering half of every planet's population in cold blood, he could instead collect all six infinity stones and give half of the universe's population swift and merciful deaths. Then he alone will be left to absorb the traumatizing experience that follows it.
"Fine.... I'll do it myself."~ Thanos

That was why he secretly sort out the power of the stones and tasked the likes of Loki, Nebula and Gamora to seek and obtain the stones, and after fully obtaining all six stones he would wipe out half the universe. No one would've known it was his cause, those left behind would assume it's all a natural unexplained phenomena and after several years of mourning the universe would make peace with what had happened and move on. But this was not the case and after much failure by his subordinates he had to step out of the shadows and handle things himself even if it meant exposing his plans and encouraging his enemies to try to stop him.
"Thanos threatens half the universe, one life cannot stand in the way of defeating him." ~Vision

The movie also gives us a similar scenario of what motivated Thanos to take such drastic actions in other to preserve life in the universe in the case of Vision and the Avengers. The Avengers were unwilling to destroy the mind stone on Vision's forehead which in turn would've killed Vision and stopped Thanos for good, but instead they sorted other means which involved safely removing the stone from Vision's forehead and prevent his death - an action that resulted in the deaths of many Wakandans who died trying to find the Avengers the time to carryout their plan. In a similar way Thanos could've easily left the universe to die and prevented the deaths of those closest to him - the likes of Gamora his adopted daughter, but instead he took it upon himself to preserve life in the universe and that meant wiping out half of the universe's population. Both the Avengers and Thanos were faced with similar situations, but where the Avengers made stupidly wrong decisions Thanos made the right one.
"The need of the many outweighs the need of the few." ~Spock (Star Trek)

Where the actions of earth's mighest heros were emotionally driven and focused on protecting their selfish interests, Thanos on the other hand suppressed his emotions and supported the logical action that needed only the strongest of wills to carryout.
"A King must know how to hurt those he loves." ~Philip (Alexander)