Memo or not, Comey's reason for memorializing the conversation was some variation of "don't trust Trump." This is a common theme. Comey also did not trust Jeff Sessions, which is why Comey and his insiders club (Andrew McCabe, Jim Rybicki, James Baker) withheld information from him.
This play is a like Cain Mutiny in reverse. Still a mutiny, and the mutineers go out of their way to torpedo the captain, but in this case the captain is sane and the mutineers are the ones talking about strawberries.