Is my black-moor goldfish killing my other fish?

I have had a black-moor goldfish, 2 guppies and a bleeding heart tetra in the same 10 gallon tank for about a year. In the last month the tetra and one guppy has disappeared. I cannot find ANY trace of them in the tank. I was told that there is the possibility that the black-moor goldfish has eaten them. Could this be true, and why now after almost a year?