Got into a discussion today about Bettas and why they can't co-exist with most other fish without a lot of space.
One opinion was Bettas are naturally territorial, higher than most other fish and therefore need the space. In addition, domestic Betta Splendens are different from their wild cousins and have the aggression ten-fold in them.
The other opinion is that Bettas are not inherently aggressive, but it is a state of mind that they get into as individual fish. People make them out to be more aggressive than they really are. This applies to both wild and domestic Bettas.