He was already involved in a plot requiring him to commit suicide, and by impact injuries and fire, at that. He was already committed to die.

He's not going to die now. That's failure.

He's not going to be a martyr. Failure #2.

And had he died, his suffering would be over, in any way that can be measured by us. His consciousness would be snuffed out, so he would have no further need to reflect on his miserable life and on his failures.

This is my usual approach to capital punishment: there's no reprieve for the innocent, should some exculpating evidence be uncovered (not the case here), and for the guilty, since I don't believe in life after death, execution is an easy out. Why curtail the opportunity to suffer? Why give him the chance to mentally prepare for a scheduled and relatively painless death? He could die of something long, drawn-out and atrocious if he's allowed to live. There's an opportunity for real pain here.

As much as I sympathize with the victims' families who sought closure through execution, they wouldn't really get it. The people they love will still be gone. They will receive a little moment of eye-for-an-eye triumph, but even at that, the criminal dies a more peaceful death than the victim, always. At the end of the day of the execution, there will still be an empty place at the table.