I don't think there's anything that says that Thanos wants "order." I'm pretty sure he just wants the universe to thrive by whatever means is most effective, not by limiting chaos or whatever.
I also think Thanos would not be against using non-violent methods to achieve his goals if it was possible, and that he honestly believes there will be a net positive to quality of living and whatever by doing what he's doing. So it's a shades of grey kinda thing as far as morality goes, closest to neutral though. Killing things doesn't make you evil by default it's about what you're killing them for. Self interest would make it evil, but trying to achieve some positive goal would make it good. Why there's no argument for good is because he's doing it against the will of the ones being "helped" and with so much bloodshed that it taints the whole thing as is.
That being said I can definitely see the argument for neutral evil. Also can see lawful evil if I've misunderstood the character and it is his belief it absolutely must be done by arbitrarily removing half rather than that just being the most efficient way to achieve what he wants ---
No problem! This is a cute and pop genocide of love!