Airline pilots definitely should not be armed. The arming of airplane pilots is likely to lead into possibly arming captains of cruise ships for an example and would definitely lead to a slippery slope of giving arms to even bus drivers. There should be caution given to arming airplane pilots.
Yes, arming pilots would not be a slippery slope, because there is clear danger on planes that needs to be addressed. Pilots face a clear danger, and arming pilots is an easy way to mitigate a great deal of that danger. It would not likely lead to other things, because the public recognizes the dangers posed to pilots, and they do not want armed individuals everywhere.