I have never had a betta that was aggressive in any way other than in the spawning tank. Thats about 5 males. They are always in a community and they are always peacful. Bettas attack fish with more of their finnage rather than their colors. I had a betta with neons about 3 weeks ago until he died choking on a molly fry a bit too large.
As much as I think its not the betta, never rule it out completely. I find that this "rule" helps in everything. I'd would say that the corpses rot quickly like I have seen with my neons when they die(dreaded ntd

).