The OP wants to know why the same process using the same drugs is described as full of suffering in terms of symptoms for criminals, but blissful when used for assisted suicide.
(And I'm not in favour of the state killing criminals or terminally ill people).
But it's not "the state killing terminally ill people". It's terminally ill and suffering people asking for help to end their lives and their suffering.
The situation is completely unlike the death penalty, where it's about punishment.
I'll agree it's not about punishment, but in countries with socialised medicine you will eventually see old people being told they're a "drain on resources".
(And I'm not in favour of the state killing criminals or terminally ill people).