Because someone less drugged up and more competent would have taken his place at the head of the Reich
After the destruction of the Nazi war machine, imprisoning him so that he can be subjected to an endless struggle session so that victims of his crimes could achieve some degree of closure by yelling at him and/or spitting on him
I'm not entirely convinced that the second option would be worth it but if that's what was decided by the community, I wouldn't waste my time debating whether or not it's a good reason to keep him alive.
The only good reasons for not killing Hitler are:
Because someone less drugged up and more competent would have taken his place at the head of the Reich
After the destruction of the Nazi war machine, imprisoning him so that he can be subjected to an endless struggle session so that victims of his crimes could achieve some degree of closure by yelling at him and/or spitting on him
I'm not entirely convinced that the second option would be worth it but if that's what was decided by the community, I wouldn't waste my time debating whether or not it's a good reason to keep him alive.