recently added 5 small rainbow fish to a tank containing 1 large rainbow fish. I have 1 small fish left. There is no evidence of illness in the introduced fish. My only explanation is that the large fish is a murderer - is this possible?
yes depending on species and size.
big male rainbowfish can sometimes get aggressive and bully smaller males and females. And rainbowfish will eat smaller fish. So if you had a 4 inch male Glossolepis incises and put some 1 inch rainbows in the tank, he wouldn't give it a second thought before eating them.
Melanotaenia boesemani and Glossolepis incisus males are the worst offenders but any big male rainbowfish can get pushy and agro. If they are kept in groups of 6 or more and you try to keep even numbers of males, and keep the males the same size, you shouldn't have any problems because they will be evenly matched.
eg: 2 males and 4 females. The 2 males can spar with each other and if they are the same size they will be able to tolerate the push and shove they give each other. But if you have 1 big male and 1 small male, the smaller one will be bullied and dominated and not do well.
-----------------------
We had a group of 2 inch long, wild caught boesemani, and put them in a 6ft tank with some 1 inch long cardinal tetras. Everything was fine for the first day then all hell broke loose. The boesemani split into 2 groups and herded the cardinals into a corner before eating them. One group of boesemani created a wall to stop the cardinals getting out of the corner and the second group swam in and ate them. We had to move the remaining cardinals to another tank.
-----------------------
When we were breeding rainbows in ponds the adults did not eat their young, and after a month we had 1 inch fish with the adults. However, any new fry that hatched out after that were promptly eaten by the 1 inch siblings.
-----------------------
As Fluttermoth said, what species and how big were they?