It's because settler-colonies are inherently genocidal. The settler-indigenous dialectic functions different from the bourgeoisie-proletariat dialectic or the aristocrat-peasant dialectic. Those other dialectics can be generalized as a master-slave dialectic where the master oppresses and abuses the slave but is still reliant on and can't truly get rid of the slave. The settler, unlike the master, doesn't need the indigenous. In fact, the settler needs the indigenous gone asap in order to steal their land and other natural resources. Therefore, the settler-indigenous dialectic can be resolved not only with the overthrow of the settler-colony by the indigenous like all oppressed groups overthrowing their oppressors, but it can also be resolved with the extermination of the indigenous at the hands of the settler. This inherent genocidal goal of settler-colonial society leads to settlers upholding genocidal ideology and in general, acting like deranged monsters.