Eureka has a mortality rate twice the national average. And you get to see the barrels they store the dead bodies in.
The entirety of "Blink" - especially what happens to the kid who hits the car; the freaky thought of a drug that strong; and the horrific come-down you would face afterwards with your body starved of electrolytes and water.
The aftermath of the Artifact in Season 2. Not only is Kim Anderson killed horrifically, but people who were in the general area of the artifact start spontaneously combusting, with only two exceptions: Stark (thanks to some timely intervention by Henry and Allison) and Kevin. And in the latter's case, there's a whole host of other changes that happen as a result.
Evil!Andy in the episode "Lost". It's amazing how chilling that omnipresent smile and happy-go-lucky demeanor can be under the right (or wrong) circumstances.