Personally for me, the most cold hearted things I have done were, as a "Raider" I went to Riverwood and killed Stump the dog, in front of the two children who play with him, then proceeded to eat the dog meat in front of them, killed the boy's father in front of him, and then used my Thane excuse to get away with it.
Nazeem deserves everything he gets, but this Riverwood thing is pretty mean.

About the meanest thing I do is boss Cicero around all day. Sometimes I make him trigger traps for me too.

Actually, he looked so livid after I sent him into a Dwemer blade trap that I've started sleeping with one eye open in Breezehome..