From Robert Louis Stevenson’s Dr. Jekyll and Mr. Hyde:
“I incline to Cain’s heresy,” he used to say quaintly: “I let my brother go to the devil in his own way.”
The reference is to Cain’s reply to God when asked about his brother Abel – “Am I my brother’s keeper?” (Genesis 4:9).
I’ve used this line at work when people ask about the progression of someone else’s project or their absence in a meeting. Like most of my answers it kills further questioning.