To some extent they are guilty of it. And to some extent they are not.

They were largely tricked into what happened to them.

What would you do, as an average orc in this situation...

You live in a society where shamans regularly speak with the spirits of their ancestors. A strange alien race (Draenai) has appeared on your planet, that are seeming to be friendly for now.

One of your leaders (Ner'zhul) comes in front of everyone, saying that the spirit of his wife has come to speak to him, and that the aliens were plotting to kill all the orcs.

Little does Ner'zhul know that the spirit of his wife is an illusion brought in front of him by Kil'Jaeden to trick him. By the time Ner'zhul finally realizes that he has been tricked, and goes back to tell the Orcs to stop attacking the Draenai, his servant Gul'Dan sells Ner'zhul to Kil'Jaeden, betraying the orcs, in an attempt to gain power.

Because of Gul'Dan's actions, the Orcs go on, under lies they have been tricked into believing to drink the Blood of Mannoroth, as their "Spirits of their Ancestors" told them to do.

Gul'Dan and Kil'Jaeden, virtually single-handedly delivered the Orcs into Burning Legion's slavery.

Why, should the common Orc, have believed anything different?

Aliens visiting their planet? Their ancestors coming to tell them that the aliens planned on killing them? What reason would they have had to believe that this could possibly have been a trick, a lie, presented to them by a third party that they had never met before?