The Manchus have a pretty complicated history. Many of the Mughal/Jurchen tribes were originally Goguryeo vassals, but they were displaced after Silla united Korea and then the newly formed state of Balhae fell to Liao and most of the various Jurchen tribes ended up becaming tributaries to both Liao and Goryeo.

Back then, it wasn't only Koreans that discriminated the Jurchen either, but Mongols/Chinese as well. The Jurchen civilization was originally at the time completely dominated and enslaved by the Khitans which formed the aforementioned Mongol/Chinese Liao empire. Before the Jurchens overthrew the Khitan, they even had a custom where married Jurchen women and Jurchen virgin girls were regularly raped by Liao Khitan envoys as they pleased.

Koreans never treated the Jurchens like this, but the Jurchens were seen just as barbaric as the Liao by Koreans because many of them were living as bandits and pirates harassing Korean communities that lived near their borders or on coasts. The Jurchens didn't raid only Goryeo either. For example in 1019, there was even a recorded case of Jurchen pirates raiding coastal Japanese villages, killing all the men, and enslaving many Japanese women as well.

At the time Khitan-Goryeo tensions were also flared up drastically and Liao ended up invading Goryeo, but they eventually ended up signing a peace treaty with Goryeo after their invading force was killed.

Then due to the shift the Jurchens managed to overthrow their rulers and turn the Mongols into their vassals after about a century, forming the Jurchen Jin dynasty which ruled most of northern China until eventually being defeated by the new Mongol overlord Temujin(Genghis Khan) who united a large number of nomadic tribes and invaded China.