Will is the substance of human existence - the force that allows for control over the behavior and thoughts of the self.
Every event that has occurred or shall ever occur in the universe is only a result of an action preceding it. Anything in the universe which appears to be chaotic - brain activity, for example - is really just incomprehensibly complex. When results vary, so too always must the variables influencing those results.
In an ideal situation in which all variables were 100% controlled - an admittedly impossible occurrence in light of the fact that the unreproducible factors of time and space must themselves be considered variables which affect outcomes - the results would not vary in the slightest.
Actions (in humans) result from decisions;.
which result from electrical muscular impulses;.
which are sent for the most part from the neural network along the spine;.
which takes its chemical orders from the brain;.
which is a complex but completely material, electrochemical organ like any other.
Decisionmaking in the brain is a function of data processing. Input (data) is received by the brain from sensory organs, mechanically encoded in chemical messages, then processed by the complex binary neural network. The resultant thoughts, decisions and actions are all output of this neurochemical process, which is not magical, not unexplainable, and certainly not the result of some ethereal influence which could be called "will." And although the thoughts and decisions rendered by these neurochemical processes are not entirely predictable, this unpredictability stems only from our current lack of detailed knowledge of the neurochemical processes of the organ itself, rather than from force ("will") whose existence is not supported by the evidence.
Not only is there not any evidence that will exists, but moreover, there isn't even a need to introduce "will" into the explanation of human thought, decision-making and behavior.