I was not talking about this case in particular but yes, of course I do think that. The method of killing makes no difference at all and to describe a method of killing as humane means to trivialize it. There are several reasons for my position. Just a few:
The death penalty violates basic human rights.
Perpetrators who are in prison are no threat to society. That means killing them is nothing more than an unnecessary cruelty, based on a medieval understanding of the law, based on the idea of revenge.
One murder cannot make up for another murder. The victim is dead and killing the murderers will not bring her back.
Governments should not kill people and in general, killing is wrong and should be avoided. Exceptions might be very certain situations of self defense, which are rare.
In countries that still have the death penalty, it also regularly happens that innocent people are executed. In the USA for example, this happens more frequently to black people, the reason being racism in the police and justice system.
Let me guess, rolling resistance is really high.