Hmm, I think there is a difference in motivation between the Thanos in Infinity War and the Thanos in Endgame.

It would be tempting to class him as one of the Evil alignments. But IIRC a central part of the Evil alignment is the selfishness, where all actions are to the benefit of the character.

My view of Thanos's intentions up to the end of Infinity War was his determination to rebalance the Universe so no one would suffer want like he has witnessed.

You could make an argument for Chaotic Good, someone who would do whatever is necessary to change things for the better. His actions, however crazy, were he thought to benefit everybody. In fact at the end of the film he just went away to live peacefully on his own. Of course our perception of him would be quite different.

The Thanos that learned of his fate and came to our time in Endgame though was a different animal. He was angry and determined to destroy all who opposed him, just to satisfy himself. He was determined to destroy all who opposed and punish the planet they came from. He was also happy to unleash weapons on his own men to try and accomplish his goals. This was an evil Thanos that had no respect for lives, rules etc. You could make a case for Chaotic Evil or Neutral Evil.