It doesn't matter if Tarrant is of sound mind or not, there is no point in keeping some who killed 51 people alive, there is no doubt he did it. This wasn't murder this was an act of domestic terrorism.
That is your opinion, not a fact. So it is fine that you are ok with killing someone by the state. But don't say that you need 100% proof because it is pretty clear you don't.
It is my opinion, just like you have your opinion.
Sure, but my opinion is based on effectiveness and humanity.
In a case like the Christchurch Massacre or Utøya Island there is no doubt who committed the crimes, there is no ambiguity, they were both caught in the act, they both admitted it, killing them would not be a mistake, there wouldn’t be any chance of either individual being exonerated at a later date.
Yes, they did it, caught red-handed and admitted it. But the point is that you are still not 100% sure that they aren't suffering of a mental defect. Or you want people who suffer from mental illness and because of that can't be held responsible, held responsible for these horrible acts. So basically omit mental illness as a factor in criminal court cases. And if that is your point of view, why not sentence a 15y/o to death if he commits a murder? Or a 10y/o? From a moral point of view, there is no difference there.
The problem with the death penalty is it’s not an effective deterrent, if you’re sentenced to death it’s needs to be carried out within a limited time frame, keeping people on death row for decades doesn’t make any sense at all, give them one appeal, they either win it or they don’t. If they don’t win the sentence needs to be carried out ASAP.
Ok, so now your point of view has shifted, now you are ok with lowering the bar quite considerably, and carry out the death penalty, Chinese style. If it is an effective deterrent you are after, fine then you need to abolish it, because it is not effective, no evidence for that what so ever. If you want effectiveness as a guideline - as it should in my opinion - than do it like your adopted country: Norway. It has an effective punishment system, much lower return rate to crime. But then you are on the opposite of the death penalty range. But the benefit is that you get a better country in return with lower cost overall to society. The only thing to "suffer" is the feeling of an eye for an eye, but then again, in the end, it will leave everyone blind.
So the only conclusion we can reach in the end: the death penalty only serves the feelings of the people left behind, but their loss can never be compensated in any way, what so ever. And the cost for society is enormous. We need to be very frank on that. So if your opinion is that the feelings of those are the most important, than yes, you can be in favor of the death penalty, but all the other "reasons" are just makeshift arguments.