I'd eat my bread without butter!
What you growlingly exclaim when told that someone did something uncaring/deceitful because "he knows which side his bread is buttered on"; what you mean, of course, is that you would rather risk displeasing the powers-that-be than harm someone else.
Back when I was a frail and tender-minded nine-year-old, my bi**hy second-grade teacher violently shook me and gave me a long severe verbal working-over merely because I had "mouthed right back" at a "spoiled-rich-kid" youngster who had been outrageously rude to me over an extremely trivial matter. At da time I was at a total loss to know why said irritable teacher had been so ferociously defensive of Little Miss Bossy 'n' Obnoxious; it wasn't until decades later that I finally learned that said bratty pint-sized colleen was da daughter of a "somebody" in town, and so I realized dat da teacher had "known which side her bread was buttered on", and therefore when said whiny miscreant went "bawlin' 'n' blubberin'" to da teacher about it, she felt compelled to blame ME for da verbal dust-up instead of just telling dat willful child to "shut up and grow up", as she should have. As I tell my friends when relating the story nowadays, though, "I'd eat my bread without butter!" before I ever unfairly blamed an innocent person like that. Reminds me of da "Little House On The Prairie" episode about da broken music box.