It's the smoke that is killing brain cells. If you just straight up eat the weed it won't, but smoking it definitely kills brain cells.
I'm assuming you're talking about Carbon Monoxide inhalation? Carbon Monoxide on it's own acts by binding to hemoglobin to prevent oxygen from doing the same. Oxygen deficiency can kill brain cells, however I'd hazard a guess that it's impossible to smoke enough weed to inhale enough CO to cause enough oxygen deficiency to cause brain cell damage.
Remember that cells are constantly dying and bring replaced anyway. There are several dangers to chronic weed smoking, dead brain cells isn't one of them. That was invented by reefer madness!
Does Smoking Weed Kill Brain Cells | smoking.ygoy.com
Signs Of Oxygen Deficiency In The Brain | LIVESTRONG.COM