Witches come in many varieties. Each witch, just like each human being, has the ability to worship whatever they desire, unless of course, they belong to a group or coven that has an established pantheon or gods for them to worship and cultivate a relationship with but this is not the case for every witch, especially a solitary one. So, please understand that the answer to this question is a general answer and does not apply to every witch you will ever meet. I would strongly suggest that you ask in a respectful way, if you are truly curious and not just trying to start an argument, the witch you meet as to whether or not he or she worships an entity known as The Devil.
First I want to establish the popular belief about The Devil. Many believe that he is a fallen angel named Satan and / or Lucifer who rebelled against God and along with one-third of the angels in Heaven, known commonly as demons, were cast out of Heaven by God for their indiscretion. They now roam the Earth spreading lies about God and tempting or making deals with humans to gain their allegiance in order to lay claim to their soul. Whether this is a story you believe or not, is really not the issue here, but to understand that there is a clear difference between Satan, Lucifer, and the Devil.
The word Satan comes from the Hebrew word “(ha-)Satan” and is a title meaning (the) adversary or (the) opposer and refers to a number of different beings whether they are men, angels or spiritual entities, who are considered evil or “bad” in that they are the adversary or opposer to God or “good”. Lucifer comes from the Latin word “lux-fer” which means bringer or bearer of light and signifies the astrological planet of Venus or the “morning star” or “dawn star”. Venus was the first “star” to shine in the evening and the last to fade at dawn. The word devil comes from the Greek word “diabolos” which means slanderer or accuser, and it was the word used to translate the Hebrew word (ha-)satan.
Now if you take time to study Christianity you will find that there are as many gods, goddesses, devils, and demons found within, just as there are in ancient paganism, however the difference is within Christianity they have been lumped together and called “God” and “The Devil / Satan / Lucifer” but some of these gods are still individual entities within paganism. Some pagans recognize, worship, or cultivate a relationship with a Horned God that Christianity would and does perceive as The Devil. Gods such as Cernunnos, (my own patron), Pan, Ammon, or The Horned God (consort to The Great Goddess) are pagan gods that are still alive and recognized within paganism though they are not, nor have they been considered evil by pagans.
Paganism doesn’t recognize strict boundaries between what is “good” and what is “bad”. These labels are subjective and relative to the situation at hand. The spiritual beliefs of paganism (Wicca included) are rooted within the natural world and it is within this environment where something can be perceived as “good” and “bad” simultaneously. The boundaries within nature are not as rigid as what Christian structure has created with the idea of God being “good” and The Devil / Satan / Lucifer being “bad”.
So, do witches worship The Devil? My answer: it depends on what you mean by “The Devil” as well as the individual witch you’re asking..