Oversimplified, and I have no idea if HSBC is worse than other banks or simply the first one on my radar at this scale, but they essentially knowing let Mexican drug cartels use them for money laundering. An example of this not just being negligence, but being deliberate, was processing transactions from blacklisted organisations by entering for example ‘na.me’ instead of ‘name’ in the wire transfer field so it would avoid automatic flagging.
By buying a Mexican bank and setting themselves up as a bank within Mexico they also acted as a way for cartels to have access to international banking, whereas previously they had been limited to the local banking system.
When investigated said ‘sorry, we will improve’, did not do so, when caught again were given an easily absorbed fine, and not prosecuted in any major way.
Knowing how many deaths are on the hand of the cartels to me this also passes some of that guilt on to HSBC, as if the cartels were not able to launder money then drug dealing would not be as lucrative (not as much use having large amounts of cash you can’t do anything with) and so less violently competitive.