Green Severums exibiting pack behavior

A few weeks ago I went out to my LFS and bought two small jack dempseys.
They were supposed to be an interesting addition to my green severum
tank. I bought them small because I knew they were agressive and the
severums are generally not so I wanted the jacks to grow up around the
severums and hopefully not bother them too much. At the worst, I figured
I would watch them closely and if the Jacks harrassed my severums too
much I would return them to the LFS.

I have five severums, all in the three to four inch size range. The
Jacks were about two inches long; so noticeably smaller (which was
intentional).

The next morning I woke up to find all five severums swarming around
the larger jack, hitting it repeatedly. It was near death but I took it
out and put it in a heavily planted quarantine aquarium and hoped. It
died two days later from its injuries.

As soon as it died I removed the other Jack which was not being attacked
but had stopped eating and put it in the quarantine tank. A couple of
days went by and it still refused to eat so I went back to the LFS and
got some live blackworms and brine shrimp and fed it those - which it
ate reluctantly. I'm now out of live food and he still refuses to eat
anything else (he even turned his nose up at freeze dried krill, which
the severums attack with gusto).

The water condition in these tanks is impeccable. I have to scoop out
the excess frogbit growth weekly to keep it from overrunning the
aquarium so the nitrate levels rarely get up to 5ppm in the quarantine
tank and consistent water changes keeps a similar water quality going in
my unplanted community tank.

Has anyone else observed this sort of aggressive pack hunting among
their green severums? Oddly enough my severums never touch the wild
caught catfish, the tiger barbs, or the pleco - but add another cichlid
and they go nuts. (They have also killed a three inch oscar in the exact
same way).

Also, what should I do about the remaining Jack? If I take him back to
the LFS he will die but I can't get him to eat!

-Daniel