Smoking pot DOES kill brain cells, because smoke contains carbon monoxide (CO). CO is a toxic gas naturally found in trace amounts in air. Incomplete burning, such as smoldering, results in CO, and it interferes with oxygen supply throughout the body. The brain is just more sensitive to oxygen deprivation than other organs. Brain-damage-wise, you're better off smoking a couple of reefers a day than the daily pack of cigarettes many smokers "enjoy". It's more accurate to say simply, "smoking kills brain cells".

The caveat: Your brain cells are dying all the time, and there's nothing particularly sinister about the effect of moderate smoking (tobacco, pot, etc.) on the natural attrition rate. If you're really into killing brain cells, alcohol is where it's at. In a neurobiology class discussion some years ago, the professor showed some slides of brains taken from lifelong alcoholics. Compared to a healthy brain, they were smaller, smoother, and visibly misshapen. Alcohol had literally eaten away much of the brain mass, more than a third in some cases.