Bring out the worst in somebody
Definition 1 of 1
Pro Tip 1/3Negative Trigger
Identify what causes a person to show negative traits using this phrase. 
Lack of sleep can bring out the worst in him.
Pro Tip 2/3Context Sensitivity
Observe that the phrase is used in situations where behavior changes in certain environments or under specific circumstances. 
Crowded places always seem to bring out the worst in her.
Pro Tip 3/3Emotional Impact
The phrase often suggests an emotional response or change in someone. 
Fights with his brother always bring out the worst in Joe.