There is no need to be intentionally cruel, but death is rarely painless for anyone. Why we insist on making a goblin's passing more comfortable than most of our grandparent's, never mind their victim's, is nonsensical.
If it has to be non-messy just use N2 to flood the chamber.
Messy execution methods are really only a problem for the workers at the prison. For the condemned it is often the fastest and least painful exit.
All that said, the biggest issue with capital punishment is that too many innocent people are subjected it, and the guilty languish so long before being executed they cost (in both room/board and endless legal appeals) far too much. Reserve it for the cases where guilt goes well beyond the "reasonable doubt" standard (free confession, in context video evidence, other clear evidence that can only point to the accused, etc), and then do it quickly. Otherwise LWOP.