What was happening could argued to be illegal and certainly from a fraud perspective because the people involved were at least defrauding each other and the system itself for the sake of their commissions and bonuses. At best it's incompetence alongside greed but it could very easily be argued as wilful fraud and/or negligence.
They first lent money that they shouldn't and they packaged debts that they knew that were crap hidden amongst debts that were okay. The credit reference agencies rated them as bullet proof because they were based upon housing and they wanted the business. As a result various other banks bought up these packages, they never looked at what was in them and the circus went on and on.
The thing that actually took RBS down was Fred the Shred wanted to expand RBS and so he bought into ABN Amro. They failed to do proper due diligence (housing never fails apparently) and ABN Amro had a shit ton of these crappy debts which ended up on the RBS balance sheet. All of the dominoes fell, RBS ran out of money and their corporate losses exceeded £25bn in 2008 and the taxpayer had to bail them out.
Would you expect criminal investigations, regulatory change and blah blah? No, nobody went to prison, some didn't even lose their jobs and post-2008 RBS bosses were given billions in bonuses and Fred the Shred himself is now living it up on a £600k per year pension!