There are irregular aspects to this case, but the idea of hitman hirers being immune from the DP is pretty crazy. That's the same as murdering someone with your own hands, both morally and legally.
This is actually not a good argument at all in-favor of the death penalty for this person but rather another great argument for its elimination entirely.
The field of neuroscience based psychiatry and the works of the great minds like Robert Sapolsky must prevail. This has to stop.
So you want to ban abortion? Assisted suicide?
Do I "want" to stop assisted suicide? No.
Do I "want" an end to organised execution of convicted criminals? Yes.
Your move.
1. There are times when doctors, in order to reduce pain in their patients, accelerate their death as part of pain management. This is usually done at the end of long incurable illnesses or injuries from which recovery is unlikely. The consequence of alleviating pain can be death. Whether it is thought of deliberately or not by the caregivers/doctors does not matter. The point is that the event of death is hastened directly by the act of helping the patient.
2. The patient is choosing to end his life. (I hope you don't ask what is to be done with depression/suicidal patients.)
... and lastly, and simply:
3. Many times the assistance may be done by a non-doctor - so the Hippocratic Oath may not apply directly.
It is troubling that there's zero evidence other than the word of a "hitman".