This is the third different video I've seen where an otter has a killed a water bird - so presumably there have been many unrecorded incidents of the same kind.
It's too be hoped it becomes more common and the RSPB take notice - they're big and powerful enough to bring some serious pressure to bear on the conservation authorities.
It'll probably get blamed on mink and sadly it's unlikely that anything will happen.
On the otter question - three really important points
1. this is not like Wolves or Beavers - the Otter population crash was only 65 - 45 years ago
2. re-introductions played a tiny part in the re-establishment (see Broughton's note)
3. they are now the Apex predator in the ecosystem and compete with us for resources (ie. fish) - but all predator populations are self-regulating over time (except humans that is).
so - it's really not a good look to be calling for culls of a native species success story, much as I regret any loss of fish stocks.
personally - I'm much more worried about the impact of increased population and climate change on fish stocks than I am about otters or cormorants.