As immune reactions are more fatal in the lungs, could inhaling peanut dust kill someone with a light allergy

Probably. But I don't recommend killing people

No I could never do that to someone. I have ethics and morals.

Everyone that I met with severe food allergies has always been thin. I assume it's because they can't eat from so many restaurants and they were forced to control which items they intake due to their allergies.

In theory, sure. In practice, I'd say it's extremely unlikely.

First, you need someone on the very high end of severely allergic. Such people definitely exist, and are probably more common than you'd think, but still not a majority. Most people with peanut allergies will just suffer from something akin to asthma: difficulty breathing, or intense swelling, but not immediately life-threatening.

Second, most people with severe allergies like this (especially the really severely allergies, like we're talking about) carry epi pens, or equivalent, around with them, which will neutralize the risk of death.