I will disagree on this one ... not that it would matter but I'll disagree anyway.
I think a life sentence is not a bit harsh, it's way over the top.
Supposedly the plane wasn't in any danger as there is a locked door to the cockpit so that passenger could have only killed or injured a few other fellow passengers.
Furthermore, how did he manage to have a knife on board? And how came he was so drunk, did he drink that much on board or was he previously inebriated?
I think this sentence is just a mirage so the local justice/government may appear to be tough on air safety while at the same time completely failing to address the real issue.
IMHO the sentence should have been a heavy fine, a few years behind bars and add a life-time ban to board an airplane for the passenger. Also a thorough investigation possibly on how he was in the position to be a threath and possibly the dismissal and heavy fines for the airport director/security manager.
As in accident investigations the last bit might be the pilot's fault but there are a LOT of other issues behind it that transformed an error in an accident.