Ack! One of my old roommates had a Nile monitor and that thing was an amazingly vicious critter.
He also had several large snakes (one, a then-12-foot Burmese python, once bit me and wrapped up my arm for about 15 minutes), but the lizard was much cleverer and meaner-tempered than any snake I have ever seen. Once it nearly took off the tip of his finger, and sat there glaring at him and drinking his blood until we pried its mouth open with two spoons. Monitors have teeth like a tiger shark. I remain a bit amazed that there is a healthy trade not only in exotic pets like monitors, but also in poisonous snakes, which are readily available over the Internet and perfectly legal to buy and own in most places.
I actually knew a guy who had two large monitors and they were sufficiently domesticated that he had them freely running around in his apartment - they would defecate in the bathtub. They are quite smart for reptiles, and can be decent pets, but to me there is nothing companionable about an animal that is that willing to eat me!