betta gone bad?

A somewhat unusual tank: 10 gallon with one male betta, one female betta and
three neon tetras. The tank used to be divided but the male and female were
spending all of their time hanging out together at the mesh divider, so I
put them together to see what would happen. No fighting, no flaring, no
nipping, and for the last six weeks or so they've been pretty tight,
sticking close to one another and swimming together almost all of the time.
They both ignore the neons. During that time her horizontal fear /
submission stripes have faded and she's become a nice light red (he's bright
red). Neither have shown any inclination to make red baby bettas.

Two evenings ago I gave them their midnight feeding and bid them "good
night." Both were happy (as demonstrated by the usual Betta Dance) and all
seemed well. Next morning at 6:00 AM I found the male betta dead with all
his fins chewed away.

This morning one of the neons was dead and also somewhat chewed upon.

She's back to grey with horizontal stripes. Remorse after murdering her SO
and pet?

My other betta pair (both dark blue, and she's nearly as dark as he is - I
haven't seen her stripes for months) is doing fine in their 10 gallon.
They've been together much longer than the red pair. Like the red pair, they
also show no desire to make baby bettas. They CAN tell the difference
between the can with flakes and the jar with bloodworms, though.

--
John Goulden