It's possible. I can't find any sources claiming Zyklon A was ever used in this way prior to Zyklon B and the first "test" killings of POWs seems to have used Z

It's possible. I can't find any sources claiming Zyklon A was ever used in this way prior to Zyklon B and the first "test" killings of POWs seems to have used Zyklon B. Most sources don't explain the differences between the various versions though, if they mention them at all. As I understand it, lethal injections are designed in such a way to paralyze the victim but the actual process is actually quite grueling, the victim just loses the ability to indicate the level of distress and pain they are in. This is unlike the euthanasia vets typically administer, which start with a sedative so the animal is unconscious, which seems not to be desired for executions. So I guess there are parallels, the Zyklon B story just doesn't serve as a contrast but rather another example: it was designed as a delousing agent but its ease of use, storage ability and relative safety enabled the industrial scale mass murder even if none of the engineers would have agreed to that. Of course to the company producing it, it was just another big government contract.