In Diablo 3, the "scary" Diablo is voiced by J.B. Blanc, who also voiced Alfred in Beware the Batman. Does that automatically make it not scary? No, it really doesn't impact it -- the result is judged by the in-game effect, not the other works done by the artist.
Who said Diablo was ever scary?
Sure, it doesn't help that in D3 he looks like a freaking Zerg, but the character has always been cheesy-looking (stereotypical big red demon with horns) and the games consistently depicted him as some sort of coward; hiding deep under Tristram, having to travel in disguise and hiding behind his horde of minions instead of taking care of things himself.
Here's a little tip for you, since you keep trying to use this to prove it's "not scary": If you can't understand what's being said, it still works.
No, it just makes you wonder what the hell was this all about.
Besides, even at the time, the novelty of a demonic figure speaking backwards had wore off and was already a commonly derided cliché. It may scare kids, but it's hardly comparable to real horror.
Imagine if in, say, Amnesia, the invisible monsters were not only visible but looked like kindergarten halloween creatures and sounded pretty much like a WarCraft Grunt.
As for the repeated taunts in D3, they happen during fights, which at this point the monsters have stopped being scary, as they are right in front of you and you are most likely kicking their asses. Even without the repetition, they are just typical boss taunts; nothing scary there.