Half of mankind will still survive after I'm through. Everything is perfectly balanced, as it should be... I hope they recall you. You deserve better than this hellhole. Maybe someday someone will learn from my mistakes.
You couldn't bear the thought of your own failure. So, where did that lead you? Back to me, I reasoned that by removing half of life, the other half would thrive. And with all life around you thriving, you wouldn't fail yourself.
Now, come here. Let me give you a gift.
Thanos declares, "I am unstoppable," which means, "You couldn't have stopped me anyhow." He was grossly mistaken since he got dusted 45 seconds later.
Thanos is known for saying, "May I?" when asking someone for something. This phrase is used by other characters too. For example, Nebula asks him, "May I have the chair?" before stealing it. Also, Thor tells him that he doesn't want to fight him since he's only a child even though they've already fought twice before. When Thanos asks why he would want to fight him again, Thor replies, "May I ask you a question?"
When Thanos attacks Avengers Tower, he says, "Where do they hide everything?" This is a reference to the movie Goldfinger where Goldfinger says the same thing when trying to break into Pussycat Central.
Another example is when Thanos uses his Infinity Gauntlet to destroy half of all life in the universe. This is a reference to Batman v Superman: Dawn of Justice where Lex Luthor says the same thing after destroying Metropolis.
Thanos' purpose is to use the stones' enormous power to wipe out half of all life in the universe, but he doesn't do it for no cause. Thanos feels that the only way to rescue the universe is to remove fighting over resources, which would otherwise result in death and misery. Therefore, his plan is to destroy half of all living things so that there's no more conflict over limited resources.
Now, some might say that this is a bad thing because it means that many people will die. But you have to remember that most deaths are due to factors that we don't control, such as natural disasters or wars. If everyone just did what they wanted with their lives, then we'd have a world full of murderers and thieves. That wouldn't be good either!
Thanos wants to kill half of all life in the universe so that there's no more conflict over limited resources. Some people may find this goal immoral, but it doesn't make him evil. He's not trying to hurt anyone.
Thanos has the ability to destroy half of all life in the universe, but he chooses not to do it. This shows that he has feelings for something or someone else outside of himself. This can also be interpreted as a form of mercy because he doesn't want to destroy anything or anyone important.
Thanos is not guilty of any crime, so he cannot be punished accordingly.
Thane is Thanos's hidden Inhuman son. After an Inhuman descendent clashed with Thanos and his army, one Inhuman lady went home pregnant with Thanos' kid. That's how Thane came to exist.
Thanos is the only known child of Hyperion and Nemesis. His other siblings include a sister named Eris who also had Inhuman abilities; a brother named Titan who used to be one of Earth's greatest warriors until he turned against them; and another brother named Uranus who has been assumed dead since before Thanos was born.
When Thanos was born, his parents feared that he would become as evil as they were. So they decided not to tell anyone about him except for Nebula, their servant. They wanted her to be the one to protect him because of her love for them. However, when Nebula found out she became enraged at them for keeping this from her and killed them. Now alone in the universe, she raised Thanos on her own.
After learning what happened, Nebula decided to teach Thanos everything she knew about fighting and using his powers. She thought that if he became as strong as she is, then no one could stand against him.
To him, he didn't just select his route; he was meant to take it. That's why Thanos feels he's unstoppable; he has to win because it's his destiny, and no one can stop him. He is unconcerned about losing or being incorrect; he simply feels that balancing the cosmos is his destiny.
Thanos believes that he is necessary in order for the universe to exist as we know it, therefore, he feels that he must destroy all life in order to restore what he sees as a perfect balance.
This might be why Thanos says that he is inevitable; he knows that he will kill everyone, but this doesn't bother him because he believes that he is doing the right thing by destroying all life in the universe.
Thanos did not choose who to murder. When he snapped his fingers, half of humankind in the cosmos dissolved at random. All of the fatalities are just coincidental. If he truly wants to choose, he'll choose all of the Avengers. But I assume that if he did so, then it would be too difficult for him to resist killing them all off one by one.