Was Vietnam actually so altruistic and just wanted to save the people of Cambodia from Khmer depredations? Or were there other factors that contributed to the invasion?
This just seems like the whole “even Al-qaeda doesn’t like ISIS because it’s too violent” which ignores a much more complicated history between the two groups.
Bringing it back to Cambodia and Vietnam, the Vietnamese would have had to suspect or know that attacking the Khmer might bring the Chinese in against them, which it did, yet they still invaded Cambodia.
Variations of this question, 'why did Vietnam invade Cambodia', tend to come up sometimes on this subreddit. I tried my best to lay out some of the context as well as address the key part of your own question, the 'was it a kind of humanitarian intervention?', in an answer I gave here: https://www.reddit.com/r/AskHistorians/comments/bpv33s/why_did_vietnam_invade_cambodia_to_topple_pol_pot/
I guess the abridged version of my answer is found in 'part two' of that response:
My opinion on the matter is that no, it was not. Human Rights violations were not a primary concern for Hanoi in the invasion of Cambodia, and certainly not the reason that the Vietnamese occupied Cambodia for a subsequent decade following the ousting of the CPK.
What this hinges on, and why I take this point of view, is that it certainly had the effect of a humanitarian intervention. But this was not the motivation.
I've heard it argued the other way many times as well, but I don't believe the evidence is there to support the claim. Khmer Rouge policies that were causing such a high death toll (on Cambodians) were not the reason the Vietnamese invaded. But their cessation was an outcome of the invasion.
Let me know if the above answer gets to the point you are getting at, or if not I'm happy to follow up with more details.