That looks like Cardina japonica (Amano Shrimps). They can get bigger, but will rarely get much larger than 2 1/2" and even that takes some time.
They will not kill anything, and only eat detrius and algae. Lots of things will try to eat them, but these are some of the fastest shrimp for an aquarium. THey can ususaly escape slower fish.
So, they will not kill your Betta. Your Betta may try and eat the shrimp. If the shrimp has lots of plants and cover, he will probably survive (and never bee seen)
Here is a link with more info...