To see many flies in a dream denotes to your conscience or unfulfilled needs. In many cultures flies are the symbols of sickness and losses. If you were dreaming about the flies that has been killed by you, then it shows the victory you will achieve through your hard work. Nothing can compete you, because you have the abilities to fight the enemies no matter how many of them there are.
Flies dream meaning
Read more about dreaming of Flies in other dream meanings interpretations.