APAD: Tarred with the same brush.

Tarred with the same brush.

This proverb means that when people are accused of something negative, others may assume that everyone associated with them is guilty as well. This is similar to being “guilty by association.”It implies everyone in a group is bad due to a few bad actors. It's about lumping people together and applying a broad, negative label to all, often seen with football fans or colleagues of a problematic person, without considering individual differences. 

This term is thought to come from sheep farming, where the animals' sores were treated by brushing tar over them, and all the sheep in a flock were treated in the same way. The term was transferred to likeness in human beings in the early 1800s.

Examples:

"He's not a hooligan, but football fans often get tarred with the same brush".
"Because she worked with the known liar, she was tarred with the same brush".