Monothelitism is a Christological “heresy” which states that Christ has two natures (divine and human) and one will; this is in opposition to dyotheletism which states that Christ has two natures (divine and human) and two wills (divine and human). Why the scare quotes around heresy? First, monothelitism was indeed condemned in 681 at the Third Council of Constantinople making it a formal heresy. However, from a Protestant position, these Councils, while helpful, are not authoritative. This means there is the distinct possibility for the council-members to make an error and unintentionally condemn something that’s not actually contrary to Scripture (alternatively, they could affirm something that is contrary to Scripture).
Is monothelitism an actual heresy running contrary to the fundamental teaching of Scripture? I think that it depends on the relation between will and nature. The motivation for the Third Council of Constantinople was to combat monophysitism: the teaching that Christ has one nature. This certainly is an actual heresy, running contrary to the Scriptural teaching of the hypostatic union. The truths that need to be held in balance are that (1) Christ is one person and (2) he has two natures. This is where presuppositions come into play.
There are two broad camps when it comes to the topic of will: combatibilists and libertarians. Compatibilists generally describe the will as a property of one’s nature. An individual wills to do something out of what her nature is. On the other hand, libertarians generally describe the will as a property of the person. An individual wills to do something out of who she is and not out of what she is . Libertarians would say where there is a will, there is a person (inb4 “where there’s a will there’s a way”).
Now, suppose that you’re a compatibilist and you hear that Christ has two natures; it follows naturally that Christ has two wills. If He only has one will, that would mean He only has one nature which is heretical. Suppose instead you’re a libertarian and you hear that Christ is one person; it follows quite naturally that Christ has one will. If He has two wills, that would make Him two persons which is super heretical.
In other words, monothelitism is a heresy within the compatabilist framework and dyotheletism is a heresy in the libertarian framework. It seems to me that in order to maintain monothelitism as a true heresy, one would have to demonstrate that libertarianism is incoherent in its rendering of wills as properties of persons instead of natures.
 This is not to say that libertarians deny one’s nature playing into one’s decisions. Soft libertarians posit that one’s nature provides the range or spectrum of choices available to an individual in certain circumstances but does not ultimately arbitrate which decision is made. This is in contradistinction to compatibilism which typically posits that the range of decisions consists of one member. This entire paragraph is a broad generalization and I recognize the shortcomings of its brevity.