Thankyou Chhiillli for the detailed response
The major problem at least guess in Ram Ravan war was different approach of various tribes. Although we believe that prior to Islamic invasions India was one (from Atak to Cuttak, from Sagarmatha to Kanyakumari) but that wasn't ever the case
India is a unique case in ancient civilizations list because of the mammoth area it's civilization covered (we know how big was Saraswati Sindhu civilization and now we find another as developed and probable Vedic civilization in Sinauli it's further excavation might change the way we look at Indian Prehistory)
People here followed different rules, had different cultures worshipped different Gods and yet connected with each other probably due to the continuity of civilization
In short India was a land of unity in diversity even in prehistoric times.
Unlike what we feel not everyone was a Vedic follower, while the lineage of Purus and the descendants of Mandhaata followed Vedas, others were not so much interested in that (I don't think Yayati was an historical figure, he in my opinion was a mythological person added later to give a common ancestory to all major tribes of the Vedas. Anus for example was definitely a Mesopotamian or Sumerian group not much to do with India most probably some of them migrated here and established a land)
Anyhow coming to the topic as the Vedic follower kings were gaining power, they started to patronise the Vedic sages all the more who now wanted to establish the supremacy of Vedas even in non Vedic lands, therefore started establishing their ashrams in the borders of areas where non Vedic followers lived. Chanted Vedic mantras and tried to woo more followers (something like what the Christian missionaries do today). This is something what the kings among non Vedic followers didn't like much and they therefore tried to stop these rituals in their land by some pouring non acceptable items into their Yagyakundas. Please check that's all they mostly did, hardly we have examples of Rakshas or Daityas killing the sages (excluding a few). This was presented as some kind of attrocities onto the sages and they asked for support from their patron kings (like Vishwamitra did from Dashrath) and the in disguise of saving their Dharma and sages rights, the Vedic partronisers attacked the kings/chiefs of those groups and killed them(it's not that they always won they would have certainly lost too, but those are generally ignored since we mostly have the texts from their side ) .
As a part of similar saving of Dharma, Ram and Lakshman had killed multiple non Vedic followers (Chhiillli listed above) and they were propagating their rituals to non Vedics even during the Vanwaas.
Ravan being the biggest and most powerful among the non Vedic follower groups (don't get confused when Ravan is called Vedic scholar because in the language of those who wrote these texts Vedas meant scriptures filled with knowledge which every group would have had for themselves. Just like we read the words like Hindu priests for Pandit ji or Hinduon ka Roza for vrat today) was definitely not happy at these happenings. Things would have had happened anyway, but the actions of Shurpnakha/Meenakshi just expedited the process..
P.S. I had once interacted with a Srilankan Buddhist online who said that they respect Ravan a lot as he had developed this region (although I really doubt if the Lanka of Ramayana is the same as Srilanka but just giving the view he gave) Aside he said that Lakshman had actually r**ed Shurpnakha and the supposed chopping off nose (naak kata di) was metaphorical (izzat chali gayi) no idea if it has any scriptural references or supports but this is what he said