When you first watch it you assume its just azmodan being a twat to leah for no apparant reason.
After finishing the game, diablos grand plot etc. etc. It's apparant it wasn't leah at all he was talking to, but rather diablo.
However, that being kinda cool doesn't make up for the general horrific act 2 story-line of "here's this boss we've been hyping since early in act 1 that killed everyone's favorite diablo character... And she's really lame, easy as shit, and there's no sense of accomplishment in killing her at all." transition into "here's the kid emperor, he's obviously belial" into "build this guy, who will so obviously betray you they don't even bother trying to hide it" into "Yep, the emperor is the guy you obviously thought he was".
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