Thanos is more powerful than any avenger/god/hero in the Avengers movies, and really evil.
He claims to be the only one in the universe who clearly sees the need to destroy half of all living beings and has the iron will to carry out this macabre plan.
It's a good idea, he claims, because then the remaining life forms will have a more comfortable life. To repeat that - he wants to kill half the lives in the universe. So the other half gets all their stuff.
Yup, he's evil. And he's wrong.