Indeed, and most of the times i need to control the urge to slaughter him the moment he starts talking. Oh well...one of these days.
I tried pickpocketing poison onto Nazeem once but since my skill was so low and he never seems to be alone it failed, of course. Isn't he essential? Maybe, maybe not. Not sure about that.
There are a few NPC's I'd like to wipe from the face of Skyrim...Delphine, that beggar at the Markarth markets and some others. One day I might just make them non-essential just for kicks and satisfaction. I've FUS Ro Dah'ed Delphine off cliffs a few times, but she doesn't care..

which takes the fun out of it.
I haven't played Diablo but I know what you're talking about from other games, but it seems to me that Skyrim just doesn't have the proper mechanics to do it properly which is because of the so called "streamling" for all audiences. Thankfully we can use mods to fix that oversight as we choose.
I guess what I'm saying though is I prefer a more balanced game, not where enemies do 3 times more damage and I do less than I should, depending on difficulty settings. On my setup I can die easily in the tutorial and even at high levels if I'm not careful with a melee character and I'm a pretty good fighter as long as I concentrate and don't lose focus...

. I like playing a mage also, but no matter which mods I use they always become overpowered early which is why I always revert to a warrior for the challenge.
I used to play Age of Conan online and that was challenging to say the least and I was pretty good too. I really enjoyed it until one time they patched the game and lots of people, including me, lost all their special and rare items as well as storage and friends lists. Never got them back either. Never played again after that and have never been interested in another online game.