Why are my guppies are killing eachother?

i had 10 fish little by little they started to disappear one by one. now im down to 4 fish. The last two that died had bite marks on them and their tale bitten off. first fish that died was the one that sucks onto the walls then the male fish that looks like a gold fish (isn't) and then the female one. Next it was the Male black guppy then the blue one which had its tale bitten off and now a female orange one. i have two boys and two girls left i think one is pregnat. And one of the guppies (male) was playfull yesterday and now he is hiding under a rock.