1) They used methods that were strange, demeaning and illegal.
2) They used methods that were strange, demeaning or illegal.
Which of the sentences should be used if some of the methods were strange, some were demeaning and some were illegal?
Which would be used if there was overlap?
Would a comma after 'demeaning' change anything.
The sole effect of a comma after demeaning would be to render the usage old-fashioned. If the methods were strange and either demeaning or illegal, I think "or" is preferable, although of course it does admit the possibility that the methods were not strange. If you really want to exclude that possibility you need to rephrase, e.g. they used strange methods that were also demeaning or illegal.