Точно известно, что Снейп был на стороне Волдеморта. Об этом говорилось даже раньше, на судебном заседании, которое подсмотрел Гарри (пожалуй, стоит напомнить):
Снейпа и оправдали именно потому, что за него поручился Дамблдор.“But Travers and Mulciber we have,” said Mr. Crouch. “Very well, Karkaroff, if that is all, you will be returned to Azkaban while we decide—”
“Not yet!” cried Karkaroff, looking quite desperate. “Wait, I have more!”
Harry could see him sweating in the torchlight, his white skin contrasting strongly with the black of his hair and beard.
“Snape!” he shouted. “Severus Snape!”
“Snape has been cleared by this council,” said Crouch disdainfully. “He has been vouched for by Albus Dumbledore.”
“No!” shouted Karkaroff, straining at the chains that bound him to the chair. “I assure you! Severus Snape is a Death Eater!”
Dumbledore had gotten to his feet.
“I have given evidence already on this matter,” he said calmly. “Severus Snape was indeed a Death Eater. However, he rejoined our side before Lord Voldemort's downfall and turned spy for us, at great personal risk. He is now no more a Death Eater than I am.”
Harry turned to look at Mad-Eye Moody. He was wearing a look of deep skepticism behind Dumbledore's back.
(IV-30)
А почему Дамблдор так доверяет Снейпу - если не ошибаюсь, Роулинг назвала это одним из центральных вопросов серии.
Но с теми словами Волдеморта, я бы сказала, не всё так просто. Чтобы не цитировать саму себя, дам ссылку на небольшое обсуждение.
http://hphoenix.h10.ru/phpBB/viewtopic.php?p=6059#6059