Or carbon monoxide - perp just goes to sleep and never wakes up. Or overdose of sleeping pills. Etc. Lots of painless ways. The reason we use lethal injection is because anti-death penalty types pushed it as humans knowing full well it is anything but hunane. The plan was then to point at botched executions to push for a ban of the death penalty or at least make it very expensive. IOW this case is an example of their strategy.
Pure nitrogen is best because it’s least impactful - and literally 80% of what everyone breathes anyway. Hardest to object to; anyone thinking that “inhumane” is just going to lead us to “yeah, whatever” and bill the family for the cartridge.