Presidential appointees are expected to be loyal to the president.

[Frederick]

Presidential appointees - such as James Comey - are expected to be loyal to the president that appointed them.
In fact, James Comey WAS loyal to the president that appointed him!  Unfortunately, James Comey was appointed by BARACK OBAMA!
Why should Trump get stuck with the baggage from Obama's administration? 
Trump's mistake was not firing Comey on the first day of his presidency.  They replace pretty much all the other appointees of previous administrations, why should Comey be an exception?  Can you imagine Trump keeping that bitch Attorney General Loretta Lynch on as attorney general?  Not a chance!

At the very least, Comey (and Paul Ryan for that matter) have been flip-flopping and dangerous to the functionality of the government.