Sorry to hear about the pistol going cannibalistic on you...
Just out of curiosity what are you feeding him and do make sure the pistol gets it?
I find it weird that he ate your other shrimp tbh...
In my 34 gallon AIO I have a tiger pistol, coral branded, cleaner shrimp and a small peppermint which I thought would die witin a day but after two months he's still alive and well.
For food i feed them shrimps which I buy in Safeway frozen and chop them up. For my anemone I feed it half a shrimp every week or so. For the pistol shrimp and goby, I make sure they both get their share and sometimes it gets very annoying because the female clown is very aggressive and even though I make sure she has enough food, she will steal the sinking pieces of shrimp and feeds her anemone. The male clown on the other hand is not like that... Guess she's in charge.
Now I think I feed a little too much because I want to make sure they all eat and dont starve, so I am fighting a little algae but in the long run I'll get that down as well.
I really hope you get it all figured it as this hobby is very rewarding once everything is dialed in and runs smoothly.