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.
+1 on @freemo - capital punishment is generally very wrong.
However of Nitrogen Hypoixia - It may be one of most humane ways to do it.
As a long time diver and Instructor Trainer I can tell you that nitrogen narcosis is a real thing... and of the "ways to exit this world" it would be in my top 10 list.
https://www.ncbi.nlm.nih.gov/books/NBK470304/#:~:text=In%20underwater%20diving%2C%20narcosis%20(nitrogen,that%20alters%20a%20diver%27s%20consciousness.