People were executed in the streets, sometimes only for being suspected of protecting jews, sometimes for petty crimes. Shot against walls.
Innocent poles were murdered for any SS officer or soldier killed by partizans, the most common ratio I remember was 10 to 1.
Mass killings, deportations, mass imprisonments in work and death camps, forced germanization of orphans and kidnapped children.
Germanization of previously polish institutions, forced requirements of speaking german.
Mass persecution of national burgeoisise and intelligentsia. Including executions, torture and imprisonment.
Settlement of polish homes, factories, farms and businesses by germans.
Special protections and exemptions for germans that were living in poland.
Encouraging ratting by rewarding people with measly protections and more food. Those people were usualy short-lived, the general population wasn't very fond of them and acted out accordingly.
Stealing of all valuable posessions and even meager posessions. Jewlery, clothing, watches, pens, golden teeth occasionaly, dollars.
Also germans were not at all reasonable people. They've often, and I mean often used intimidation tactics to get what they want.
Dehumanization, destruction of personal and private property including sometimes seizure and theft, death threats if not outright murder of relatives of the person they were intimidating. Forced imprisonment and torture of people who were only suspected.
Eitherway, if you got me a Nazi tied up to a pole and told me "Go to town.", you'd see me do things I would even be horrified to look at.
EDIT: I also forgot using civilians as human shields during the warsaw uprising and other similar actions. But the uprising is the most popular example of that.
Oh and raising warsaw to the ground.
To. The. Ground.
Not even the Soviets could imagine a horror like that.
A whole city and it's history, wiped from the face of the earth.
EDIT 2:
OH MY FUCKING GOD HOW COULD I FORGET ABOUT THE GHETTO. FFS