Unfortunately, I can see both sides, but then, I have always been a bit of a fence sitter. Since we all know there have been cases where the wrong person has been convicted of a crime, the Death Penalty is a touchy subject. Add people's versions of morality, religion and the like and you have a real powder keg.
With this guy, it is not just the weight, but his health and the medicines given to him to improve his health that come into question. If, indeed, the drugs he takes will make the injection that puts him under ("to sleep") before the lethal doses are administered ineffective, then yes, it is inhuman.
Yes, I know the argument about "is it any more inhumane than what he did to his victims?" but..two wrongs don't make a right.
If, in the legal system, he has been condemned to death, being "too fat" should not come in to play. After all, there are other drugs that can be used to achieve the same results as the most often used one. Whether the law is just or not, why should he be treated any differently than any one else under the same set of laws?