Colonialism is definitely a unique sort of evil. If you don't think that I'm sceptical that you're particularly broadly read.
The scale of atrocities committed is just beyond comparison, like the most hardcore genocide in ancient times (that we have evidence of) was against a single city. In colonialism we are talking about continents.
So genocide is the key word. A lot of conquest did not involve the displacement or extermination of entire peoples until colonialism.
Conquering a place was largely "so you pay tax to me now" prior to colonialism, at which point it became more "so you are subhuman and need to be exterminated or bread of of existence"