Offs,given that this has been posted in 'parenting' thread and the site is called 'Mumsnet' I think we can allow the op a bit of slack!!
Op, my mother is/was also cruel and sadistic. Her father was the same.
Examples of her behaviour to us would be;
Burning the back of our hands with hot tea spoons after stirring a cuppa 
Putting me in the bath while I was having a toddler meltdown and turning on the cold tap 
Smacking us with slippers, dog lead, shoe on bare skin or her favourite was to do it while we were in the bath - wet skin made it much more painful 
Humiliation in front of friends
'Accidentally' playing rough with us resulting in us being hurt. This is something I noticed her doing and watching her play with my son in the same way set alarm bells off.
She was never allowed to babysit him again until he was older.
We now have a very low contact relationship, she sees no wrong in how she was. If we mention any of her smacking behaviour she cries and acts like the victim.
If it helps, I have never ever had any desire to hurt or humiliate my child.