It is not only illegal, but it is a crime. Although the peculiarities of such a crime depend on the given law of the applicable territory, by principle in all (at least democratic) jurisdictions, refraining someone of the possession of his/her property is deemed to be a crime. The debate, whether bitcoin is money, commodity or something in between is not relevant here. The fact is that someone is its owner through regular means of obtaining such property, without damaging the position of others.

Criminal Law requires the existence of actus reus and mens rea in the case. What i have described was the actus reus, which is taking over the control of property without the will of its owner.

Now, mens rea means the mind of a criminal. That means that the person who perpetrated the actus reus was doing it willingly, presupposedly knowing about the consequences and presupposedly knowing that such an act was against the interests and the will of the owner of the property.

Therefore it does not matter how Bitcoin was taken from someone's wallet. Even if the private key was negligently on the owner's work desk, it is a crime using it for anyone else, without the permission of the owner.