Voldemort and the Death Eaters couldn’t produce Patronuses but they didn’t need Patronuses to protect themselves from Dementors. As Voldy himself says, Dementors are their “natural allies.”
This means that Dark wizards are more or less desensitised to the effects of Dementors, which indicates it was pretty useless employing Dementors to guard Azkaban as they didn’t have any ill effects on truly evil people.
Which in turn means that only average criminals in Azkaban who had some decency in them were affected by the Dementors—effectively punishing people for having some good in them. How sick is that?
This point really isn’t a matter of debate either. This is made clear both in canon, and in Rowling’s post-canon interviews and writings.
There is only one major inconsistency, and that is Umbridge’s Patronus. She shouldn’t be able to produce one! Full stop!
Although generally Dark witches and wizards will not try to produce a Patronus, not having any need for one, most Dark wizards will be devoured bycoming from their and consuming the caster.
For this reason,and his were not able to cast a Patronus Charm (not that it was necessary, as they already had the dark creatures under their control via their affinity to darkness, and would have no use for Patronuses).
However, some witches and wizards of questionable morals, such as, are able to produce corporeal Patronuses, Umbridge doing so despite the fact that she was considered an evil person; her wearing of , which was one of Voldemort’s , was what enhanced her strength due to her evil affinity. She was able to produce a corporeal Patronus due to the enhanced strength.
Conversely,was unable to cast one despite his waning evil. Severus Snape is the only Death Eater capable of casting the charm, as his love for was his one redeeming point.