Tolkien against euthanasia

'He will not wake again,' said Denethor. 'Battle is vain. Why should we wish to live longer? Why should we not go to death side by side?'

'Authority is not given to you, Steward of Gondor, to order the hour of your death,' answered Gandalf. 'And only the heathen kings, under the domination of the Dark Power, did thus, slaying themselves in pride and despair, murdering their kin to ease their own deaths.'

- The Return of the King