He felt so sad about the firing of Jim Comey, you see.
Rosenstein was “overcome by emotion” when discussing Comey’s firing during his interview with FBI agents “but quickly recovered and apologized,” according to a summary of his interview.
Then, after admitting that he knew the purpose of his memo was the justify Comey’s firing, decided to hire Muller as special council over it:
On May 10, Rosenstein called Mueller — already thinking about appointing a special counsel. Rosenstein had learned from an FBI briefing that Trump was not a suspect and said he appointed Mueller due to how the public would react to the firing.