Is your objection to execution by gas, or execution at all?
I agree that the death penalty is wrong. But if you are going to have the death penalty gas is probably the most humane way to do it.
Most AR-15 in the USA are used for small game, it isnt even powerful enough or suited for large game. Frankly, its just far too weak of a rife to be the best choice here, you'd want somethign more powerful to be humane.
Huh? The majority of school shooters dont use AR-15, likely for this reaason. Moreover most school shooters have never owned a gun before (by virtue of the fact of being a kid) so they arent picking guns because they are the most effective at killing, they are picking them because they are the only one they can get their hands on (and small-game guns are the most common in the usa due to being the most affordable)