You can not blame Petco for one fish killing the other, the clowns get territorial like other fish do and that is what made the fish beat up the other. One clown needs to be dominate over the other this way they may change sexes and you will have a male/female in the tank.
Add more LR
so there are plenty of places for all the fish and critters in the tank to have a place to hide.
Anemones need mature tanks, what kind of light do you have.
Get 2 of the same type of clown and get them small so they have a chance to turn into a pair.. Add them at the same time, and take your time when acclimating them, poor acclimation can weaken fish.
What happened to the winning clown??