He could produced a Patronus because he knew what love was. Love is the most powerful positive emotion ever (no matter if that love is painful or not). Therefore, Severus was able to produce it.
Honestly, can you imagine Lucius Malfoy or Bellatrix Lestrange producing one ?! They just mean it when it comes to make suffer.
Severus can conjure a patrounus because he's not really a deatheater. Deatheaters can't conjure a patrounus (I assume) because they have no good in them, no happiness, and no love. Only greed and ambition.
Yeah exactly LilyZ~!
Also i reckon cuz Lily was a source of happy memory in alot of ways while i think the Death Eaters on Voldemorte's side in some ways lost all their true positive-happy feelings yknow...
To produce a full Patronus you have to full your mind with happy things, with the persons you love, the things that makes yous happy. Severus thought about Lily but the other Death Eaters didn´t had nothing "happy" to think about.
I think he waz a death eater cuz in hp 4 serious tells Harry tat a death nevever quits being one...
Bt the reason he could produse a patronous waz becaz he had happy memories whereas the other death eaters happy memories were clouded by fear of voldemort and cruelness.they could hav had good memories bt tat would hav been of torturing or killing someone so they really cnt be happy memories...
Severus could produce a patronus because he loves. When producing a patronus one must think of something which brings a happy feeling. Death Eaters do not understand joy and happiness. Dumbledore said " Severus is no more a death eater than I am"