A guillotine is a bit after the medieval period.
A good old fashioned headsman's axe and a tree stump as a chopping block will do the trick nicely.
I would rather hang them though, unless you are in the former holy Roman Empire, beheading was considered a more dignified death.
In the HRE, for some reason, it was viewed as a terrible death.
Hanging being reserved for peasantry, and for crimes like poaching, sheep rustling, other such low born misdemeanors.
I will check what the preferred method of dispatching saracens and moors was though.
Places which mistook dark skinned men for Lucifer may have just outright burned him.