My understanding is that the death penalty process is kind of a hack job and can't be done properly because no medical personnel will participate in it due to the Hippocratic oath. This is not a concern in consensual euthanasia.

There's also an issue with the death penalty process seemingly requiring suffering, as evidenced by the lack of use of inert gas which appears to be painless considering accidents with poorly-ventilated spaces where people unknowingly pass out (and sadly those who go help them suffer the same fate). Even in states where the "gas chamber" is a thing, cyanide is used instead of just inert gas despite it making the post-execution cleanup process safer (so would make sense even if you did not care one bit about the suffering of the condemned).