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Source: https://writers.stackexchange.com/a/41335
License name: CC BY-SA 3.0
License URL: https://creativecommons.org/licenses/by-sa/3.0/

1. A species of savages are unlikely to be a space faring race

A group of intergalactic savages would likely be a subgroup of an advanced alien species. For example, the space pirates of an advanced species, savages among a highly evolved species.

1. What you are describing is cultural differences. In old America, the smallest argument could be cause for a duel (maybe your aliens just like to settle things with a duel). In Bushido Japan, a Samurai had the right to cut down any non samurai peasant. (maybe your aliens just see humans as peasants and cutting humans down for any slights(real or perceived) is wholly justifiable.) Or maybe your aliens are space religious zealots. They are just converting the galaxy by blaster fire. Or they aren't like humans at all. Maybe they have a hive mind... They just want to expand to Earth, and the humans have to go (but that doesnt seem to be the direction you are going)

And how you would write them without them coming across as SAVAGES, despite their actions? EASY! Just write some scenes from their point of view. Remember all great villains are heroes in their own story.

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Expanded to take into account of the OP's comment.

I can't read your mind. If you are just talking an alien, non human and non-space faring species, on a different planet, and you want to describe them to a human audience..... then you must do a few things.

1. You have to describe their habitat. 
2. Their biology. What are these creatures. Are they like ants? or more like some kind of earthly mammals? or more like fish in the ocean? how do they reproduce? Sexual? Asexual?
3. what is their culture like. how do they communicate? Do they have a language? Are they mind readers, where every is an open book to everyone else within a certain distance? Are they hive mind?
4. What are the important resources to their survival?

without a lot of world building information how can we tell if they are savage or not?

Original score: 2