How do you know they are no longer a threat. How many murderers have been released only to kill again. This lad both planned and boasted about what he was going to do and has shown no remorse, he actually said job done after he did it. This was premeditated murder not an accident spur of the moment or self defence. He knew exactly what he was doing so surely he knew what the consequences would be in fact it was what he wanted.
The teacher wont come back in 20 years and her family have to live with this for the rest of their lives.
I am not in favour of capital punishment across the board but in the case of premeditated murder proved without doubt then I think this should be the case.