Haruo committed suicide by Godzilla to ensure that all traces of nanometal were gone, not purely for spite as you suggest
Yes! But not only, Haruo did "sacrifice" himself too because of King Ghidorah influences, at the first moment Haruo saw the Nanometal at the end of the movie he instantly knew humanity would try again to get revenge with Godzilla and start the cycle all again which ultimately leads to destruction and the (in)evitable fact of King Ghidorah reappearing and collecting his desired "fruits" (which funnily is more literal than it looks like, Ghidorah would literally devour the planet, hence the title). It's very justified, not something they came up with at the very last moment.
Anyway, I enjoy the anime trilogy quite a bit, very grateful for its existence.
Can I just point out people sometimes miss things on first viewing and don't catch till second or third and as a lot of people didn't sit though repeat viewings they may have missed stuff
Exactly this, the movie aren't as heavy like EVA NTE movies regarding to content or information but there's quite a lot of lovable stuff you can't catch on the first viewing. I would recommend reading the fanservice (there is more fanservice than the entirety of KOTM and Shin!) filled novels to catch some worldbuilding on the movies, but unfortunately at least when I was active in the Godzilla community (not so much time actually) it wasn't yet fully translated to English and there's no hope it would ever be translated through official means, but there's a cool Wikizilla video about it(this is something I would like to be grateful about the EVA community, there's so many delightful translated things here it's almost unbelievable, unfortunately the Godzilla community doesn't have that ).