Haha, I remember sending you a stressed PM when I got my first guppies because I thought Sam was terrorizing them. I've noticed with my other more active boy when introducing a endler that he chased, flared and as soon as the fish was out of sight completely forgot about him. I honestly think it's a shame bettas get such a bad rap about being merciless killers when most are simply curious. I never had a hunter betta or really seen any massive aggression at the LFS where they keep bettas in community tanks, but it probably depends massively on the personality of the fish along with how threatened they feel about 'invaders' in their space. Bettas often remind me of cats in their behaviors. Very territorial little fish with massive personalities.
I'm sure space in a huge factor in aggression, too. My betta is content to ignore all his tank mates in his tank because he has his own space and side to chill where no one bothers him.
If tank mates harass betta, say a tetra is getting too nippy, would a betta begin lashing out? Or ignore it? Depends?
On a side note, I've heard when picking out a betta you can get a better guess on the temperament and aggression of the fish by setting two cups next to each other, and the more submissive fish will be better in a tank. Aren't bettas smart enough to know betta from say, corydoras?