I keep saying it, it might feel bad that Ozpin kept lying to characters we like, but time has proven to him over and over that when people learn these things, they turn on him or do bad things with that knowledge.
>Hides the truth about Maidens because people kept hunting them down to steal their power like Cinder and Raven.
>Hides the truth about Lionheart because literally nothing good would come from people knowing that a faunus in a high position of authority cooperated with the Whitefang in a terroristic attack.
>Hides the truth about Salem because nobody wants to fight someone they know they can't kill like Raven, Lionheart, and maybe even Qrow and Wiess now.
>Hides the truth about the Lantern because someone might use it to find out any of the other terrible truths he's hiding like exactly what Ruby did.
>Hides the truth of what happens someone gathers all the relics because the gods will literally destroy humanity forever and someone like Salem may think that's a great idea.

Jaune stabs Salem though the chest because he didn't know about the no killing Salem bit because no one told him

Salem dies

Jaune: "Look I killed her, it only took what seven seconds, ten tops."