What is the worst thing a blood clot in your leg/calf can do to you?

Unfortunately death: Blood clots in the legs can grow to cross above the knee and eventually break apart and go to your lungs. Depending on the size of the clot that goes to the lungs you can die from this event (it is called a pulmonary embolism). Clot in the legs can also cause significant pain and swelling in the leg.

Kill you: They can travel to your lungs and cause pulmonary embolism, which can be fatal.