[SPOILER ARLERT: Don't read this if you haven't read the scans and don't want to be spoiled]
I think we can start by saying Isayama is a nationalist and I've heard people say the way he draws Titans is somewhat anti-semitic.
The lesson of this serie also seems a bit reactionary, the fact that "walls work" for example, even if they end up not working so much in the end.
The genocide versus self annihilation aspect is also to be discussed I guess. On the subreddit, you can see many people that are pro Eren, pro destruction of the entire world. I guess if one thing can be said about this series is that there are really no actual good guys, everyone is a killer and all factions suck; whether that's good depiction of politics or not, I couldn't say.
I'm not sure that I would say that SnK portrays human nature accurately. His character's are more like tropes with giant expressive eyes. And I really do think there needs to be more discussion about the screaming/crying boy trope in anime. The older I get, the more insidious it becomes. I can listen to all manner of nonsense in a normal tone, but screamed nonsense, the way anime tends to deliver it's surface level observations, is instantly grating. It also implies that the worlds problems can all be summed up by a screaming teenaged/preteen boy. Good fan service, I suppose. But I doubt thats the most important message the authors are trying to communicate. And with Isayama's stated admiration of imperial Japan (Mikasa was an Imperial Japanese warship and Dot Praxis was modeled after an Imperial Japanese general ) and the Korean occupation, his themes of constant,gruesome struggle take on even darker undertones. Becausee of this, China has banned it and Korean death threats to Isayama were frequent.