Male bettas are the "baby sitters" and they are the ones that look after the eggs once they have been fertilized. The males make an excellent baby sitter, replacing any eggs that have fallen out of the nest (as the eggs are heavier than water, they sink if they fall out). On the other hand, the female, once she has laid her eggs, doesn't want to know and is quite likely to eat them. This is the reason that the male may kill her - he doesn't want his offspring to be eaten before they hatch. Once the eggs have hatched and the fry are free swimming, the male must be removed otherwise he'll eat the fry
(He's allowed, she isn't - typical male chauvanistic behaviour
) Another reason that they're kept in small containers is to show them off. When a male meets another male, they first flare their fins at each other and then fight, sometimes to the death. When they are in small containers, they can see other males so they flare, making them look "pretty" for prospective buyers, but they can't fight. They can be kept in a community tank (mine lives happily in a 45 gallon with sailfin mollies, zebra danios, and a load of different kinds of bottom feeders - he's been there a long time now and is extremely happy) but the tank should be a well planted one with lots of hidey holes. Personally I don't agree in keeping them in vases, cups, jars or anything similar - I think the smallest should be a 2.5 gallon tank. (which would you prefer - a one room bedsit or a 5 bedroomed house with garden?
) BTW, this is an explanation how they breathe air - the labyrinth organ is situated on either side of the head in the gill cavity directly above the gills. The fish takes in air from above the water surface and passes it into the two chambers of the labyrinth, each of which has a rosette like structure and looks similar to a sponge. This "sponge" has a very good supply of blood vessels and the air is absorbed straight into these vessels.