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Nightmare Fuel: Iron Man: Armored Adventures
Rhona tried to gas Whitney, cut Rhodey in half, vaporize Pepper and electrocute Happy.
And she did something similar once before, so she probably has quite the bodycount behind her.
Mallen, once he takes the Extremis formula. The creepy cocoon he comes out of, his black eyes with red irises, his decomposed skin, the growing insanity, not to mention his enhanced strength, super-speed, and lightning and heat powers, make him one of Iron Man's most terrifying foes.
There's also the sheer insanity in his actions.
Pretty much everything about Technovore.
MODOC itself is pretty scary. It's a technologically advanced Humpty Dumpty that, to put it simply, Mind Rapes you and steals everything in your head.
You didn't misread that, people. This show actually manages to make MODOC legitimately threatening.
Also, this version of MODOC looks just plain creepy. It's especially unsettling because because this was never a problem with MODOC.
Justin's beginning as Titanium Man has him brutally blow up Iron Man's empty armor, and actually express disappointment there are no blood splattering everywhere.