But the severity of the crime doesn't dictate what medical treatment you receive
Should a murderer on death row not be allowed insulin or dialysis because they're a murderer?
We have to accept some amount of mental anguish existing in our death row and life sentence prisoners. Being on death row or being sentenced to life without parole are going to cause a lot of mental anguish.
Give them insulin to keep them alive, yes. Mental anguish is a given, though, and I can't imagine many people are losing sleep over it.