First off if he hasn't been convicted then he's entitled to presumption of innocence. I know you lose some rights when on remand etc but not all of them.
Given that there is a euthanasia law, and given that he meets the criteria, the system acknowledges that he is experiencing 'unbearable suffering'. So to remove the right to choose euthanasia would be to condone the use of unbearable suffering as a form of punishment within the judicial system, which is essentially torture. The punishment for crime is deprivation of liberty, one forfeits the right to freedom. I'm not convinced that one should forfeit the right, if it exists in that society, to bring an end to one's own unbearable suffering.
Another interesting point is that I'm sure there will be some who would support capital punishment for a crime such as he is accused of committing. I'm not one of them, I'm against it, but if such a sentence were to be carried out, I believe it should be done in as humane and painless way as possible, which is what is suggested by the term 'euthanasia'. Here the punishment is loss of life, not undue physical suffering.
So under both of these arguments, forcing him to endure 'unbearable suffering' with no prospect of it ending does seem unnecessarily cruel.
Of course it would have been better for him to stand trial and there should be a thorough inquest.