Can helium kill brain cells?
Not immediately. Helium has many negative effects on the brain. Bubbles of helium can travel to the arteries that lead to your brain. Inhaling the gas won't kill brain cells on its own, but sucking it in can cause oxygen deprivation, which can lead to your precious brain cells slowly dying.
I don't think helium directly kills brain cells, because it is an inert gas. However, it can lead to brain damage because it replaces the oxygen in the blood. The lack of oxygen is what can lead to unconsciousness and even death. Breathing a little helium from a balloon followed quickly by breaths of fresh air would not cause brain damage, but repeated breathing of the helium would be extremely dangerous.
No. Hand sanitizers on your hands just kill pathogens (Pathogens is another word for germs, if you don't know what it means). The pathogens can't evade the alcohol. If they do, they may kill your brain cells. I do not know about other alcohols, but of course, it won't kill your brain cells if it doesn't have to do with killing germs. That was a very good question, anyway.