In addition to the reasons given so far, remember that Thanos is a true believer in his cause. He doesn’t see himself as some power-mad supervillain but rather as the hero doing a regrettable but necessary task to prevent greater suffering.
Thanos: Little one, it's a simple calculus. This universe is finite, its resources, finite. If life is left unchecked, life will cease to exist. It needs correcting.
Gamora: You don't know that!
Thanos: I'm the only one who knows that. At least, I'm the only one with the will to act on it.
Thanos is also quite explicitly not trying to save the people he cares about. He’s quite adamant that the process must be completely fair
Thanos: Titan was like most planets. Too many mouths, not enough to go around. And when we faced extinction, I offered a solution.
Dr. Stephen Strange: Genocide.
Thanos: At random. Dispassionate, fair to rich and poor alike. They called me a mad man. And what I predicted came to pass.
It would go against everything Thanos is shown to believe in if he was to try to get around the trial by forcing someone else to make the sacrifice. In his mind, he is the one person who can make the tough but necessary decisions and as such has a duty to do so.
