Inmates that have got the death penalty and that are killed often dies in a way that is against the US constitution. http://medicine.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pmed.0040156 Do not expect any changes though, a state that is barbaric enough to kill people will not care how the inmates die.
It does seem to be a bit barbaric to me - the trouble is that the other methods are equally as unhumane (or can be). There's a good website (The Death Penalty Information Center) that tells you about how some executions don't go according to plan. It also has more about the consitutional issues to which you refer. Worrying stuff.
Some people deserve a painfull death in my opinion for what they've did .... For exemple rapers or pedofils i shout them without remorse