https://www.nytimes.com/2018/01/25/us/politics/trump-mueller-special-counsel-russia.html?mtrref=undefined&gwh=5E47A77F08D35F8B935B357CF108EF41&gwt=payOh boy
Trump wanted to fire because:
1. Mueller left a Trump National Golf Club club over membership fees
2. Mueller most recently worked for a firm that represented Kushner
3. Mueller had been interviewed (and failed) as FBI director the day before he was appointed counsel
Trump also considered firing Rosenstein and putting Rachel Brand (No. 3 at the agency) in charge of the investigation