English Translation
The man who I thought was my friend deceived me.
Why isn’t this “disappointed” rather than “deceived?”
he deceived me = er täuschte mich
he disappointed me = er enttäuschte mich
“Enttäuschen”: literally “Ende des Täuschens”
like
You think somebody is nice, but he deceives you. You find out and then you are disappointed.
So in German disappointing is the end of deceiving.