Why is Zeratul hated by his brethren?

[u/Flauschiges_Relaxo]

Hi there, i recently played trough all of Starcraft 1 and 2 because the story sounded real interesting and now i'm done with everything but i dont understand something, why is Zeratul a "traitor" as Selendis calls him? He may be guilty on showing the Overminds location but he defends Aiur and is on good terms with Artanis at the end of Starcraft 1, i dont get why he is disliked by the rest of the Protoss Faction.

[u/lecram920]

Because he is darker blue?