Everyone! thanks for your amazing response!
From the sound of it, looks like betta personalities are hard to predict, though safe to assume they would be aggressive. It baffles me, because my sister's 2.5 gallon tank houses one betta and 4 amano shrimp, and nothing is happening!
Funny thing though, later that night when i removed the shrimps from the tank, I caught the betta nuzzling into the amazon sword plant, like he was making it his bed. This is now his preferred spot to rest.
So now it got me thinking, maybe he was attacking the shrimp because he wanted the amazon sword spot for himself. The shrimp also loved it there, because it was the biggest, and most dense aqua plant. Anyway, I figured this meant planting more dense stem plants and the like, perhaps it would placate the betta's aggression (which may have been linked to territorial preferences).