Immediate help with a Betta and Sunburst Platy?

I had a Blue male betta fish for about a year now, and I've noticed lately that he hasn't been swimming around the tank and just kind of floats in the same spot all day. I went to the pet store, planning on getting some new plants to liven the tank up, maybe cheer up the fella. Well anyways, the sales man suggested I buy 2 Sunburst Platys, but once I put them in, the Betta started flaring his gills and attacking them. I thought those fish could co-exist? Is it something that will take time for the betta to get used to? Or will he try and successfully kill them both