How can I kill Kerbals? From what drop height can they not survive?

The Kerbals are a very weak species and will die quite easily. I find that 50m is usually a sure death. They can also die from an explosion of the command part (which varying on the part can withstand different blasts).

I found the Kerbals to be both flimsy and durable depending on the situation.

Command module falling over: Kerbal dies in the explosion, despite the fall of the module not being that great to begin with, just enough to break the module.

Reentry: This happened not long after starting the game... I decided to try an EVA with jebediah, not realizing that i wasn't in a full orbit. As soon as a realized that i was on my way down, fast, without any means of getting back to the plummeting ship, i proceeded to fire the EVA suit at full, hoping that i could slow down and come to a nice landing. Of course the EVA thrust wasn't big enough, but it turned out that the EVA suit and the drag combined slowed me down enough to actually have Jebediah survive a fall from 90km. The landing looked painful as he bounced off the ground, but BadS as he is, he just stood up afterwards, grinning, awaiting recovery.

Construction accident: A kerbal fell down from the top of a 12-stage rocket ready to launch. Hit a few parts on the way down, and faceplanted into the ground. Coworkers were unable to revive him, and his life insurance now pays for the R&D departments coffee-consumption.