Sink so low
Definition 1 of 2
Pro Tip 1/3Emotional Impact
Conveys deep disappointment; used when someone's actions are shockingly negative. 
It was sad to see the respected politician sink so low as to accept bribes.
Pro Tip 2/3Unexpected Behavior
Highlights actions that break from someone's usual moral standards or behaviors. 
The community was stunned to see the charity founder sink so low as to embezzle funds.
Pro Tip 3/3Moral Judgment
Used to express a negative moral judgment about someone's actions. 
How could a teacher sink so low as to falsify their student's grades?