As long as executions are legal, there should be no exception for the insane. If people deemed insane are not subject to the death penalty, it opens opportunity for injustice and corruption. Those who would not traditionally be considered insane can easily manipulate sanity tests to avoid the punishment that other criminals of equivalent crimes may be killed for.
First off, I have never been a believer in the death penalty. Killing people to show people that killing people is wrong doesn't make any sense to me. I don't believe that sentencing someone with mental incapacitates should be allowed. If someone is truly mentally incapacitated and they have committed a crime like murder they should definitely be given life behind bars, but not sentenced to death because they don't know any better.