Northern Rock, collapsed because of bad mortgages.
Global crash, caused by bad mortgages.
Yeah, totally different.
Credit crunch caused by lack of availability of the existing cheap, dollar based lending used to capitalise banks.
Problem accelerated by the fact that a lot of these loans had also been used to make dollar investments in US asset backed securities.
When the short term borrowing suddenly dried up, the banks ran out of money very quickly. Hence the Federal Reserve lending money to central banks around the world to then lend on to the banks to bail them out. And, in the UK at least, several banks being nationalised as part of that (note the government didn’t provide all of the bailout).
So, no, flaky mortgages didn’t cause the global crash. They just helped it to happen faster.
And, no, national debt still isn’t a bad thing.