Comey knows he can go to prison if this is all laid bare. He needs to take out Trump to protect himself.
This is how the uber-corrupt do it. They don't take bribes in suitcases or offshore accounts. They are given multi-million dollar gigs at cooperating and colluding corporations (Lockheed, HSBC in this case) in between gigs as US Atty etc.
The upside of this is that Comey will throw Lynch under the bus if he has to save his own hide. He was worried about Arkanicide during the Obama administration if he talked then; now he's got to weigh that risk against the risk of prison.
Don’t forget HSBC was at the center of scamming the global financial markets by manipulating LIBOR - the interest rate that is the basis for deriving most of the lending rates.