"There's got to be individual responsibility here," he said - renewing a warning to the bank made by Chancellor George Osborne earlier this week.
"It's one thing to fine an institution, but an institution is made up of people who in many cases better themselves by behaving badly and we've got to sort out where ethical misbehaviour leads to sanctions. It is bizarre to us people as to why activity that most people would believe is fraud is not pursued."
The lender is one of about 20 banks being investigated over involvement in manipulating the rate, which governs the price of more than 500 trillion US dollars' worth of loans and transactions around the world, including household mortgages.
RBS's fine is set to dwarf the £290 million settlement agreed by Barclays last year over its involvement.
On Tuesday it emerged RBS investment banking boss John Hourican, who was widely expected to shoulder the blame for RBS's role in the interbank rate-rigging affair, is to step down. He is reported to be in line for a parting gift of a year's salary in lieu of notice, worth around £700,000, but had been asked by the bank's board to forfeit the £4 million he is owed in shares.