Why does macbeth have assassins attempt to take the lives of banquo and fleance?

Certainly because they were his threaths and silent enemies. THis is a beautiful Shakespear story why dont you read it for yourself to know - you will not regret .. but you need a broad mind and understanding bec the expressions (for me) are done indirectly -- simply because it was a play in the old english days. Its worth reading anyway as long as you will understand it along the line.