#2 To project Drew McIntyre as a sadistic monster heel
Even though he had to spend some ridiculous amount of time aligning with random heels of RAW to battle the Shield, Drew McIntyre is still one of the top heels on RAW.
Once Brock Lesnar says goodbye to WWE for good, they may look forward to ascending the Scottish Psychopath as the top heel of the company on the Red Brand.
Thus, to make him look credible, WWE had to push McIntyre to destroy a top babyface of the company - someone like Dean Ambrose.
Since Ambrose was already on his way out of the company, there was nothing for him to lose anyway. Hence, WWE decided to have McIntyre brutally destroy Ambrose - a classical method of a departing Superstar giving the rub to someone who needs it.
Edited by Alan John