If an event is said to be random, it is not caused or predetermined by any other event. It simply came into being sporadically. If an event is not random, it is caused by some other event prior to it. This prior event is itself either random or not random.
It is obvious that every event is either random or not random. Lets suppose you want to make a decision. If that decision is random, obviously you had no free will (as free will implies choice and not just random chance). If on the other hand, your decision is not random, then it is caused by some other event. If the other event is random, then again, you have no free will. If the other event is not random, it is caused by some prior event. Repeat this process ad infinitum...
In short, every event is either random or predetermined by the universe. Free will does not exist?