This article explains better than anything else why he's doing all he can to take down Trump. Trump asked him to write the letter for the Comey firing and he has resented it ever since.
"Sarah Isgur Flores, disputed the accounts of Mr. Rosenstein’s behavior. If he was angry in the days after Mr. Comey was fired, she said, it was because Mr. McCabe concealed from him the existence of memos..."
nytimes.com/2018/06/29/us/…