I was not talking about this case in particular but yes, of course I do think that. The method or killing makes no difference at all. There are several reasons for my position. Just a few:

The death penalty violates basic human rights.

The perpetrators are in prison and therefore no longer pose a threat to anyone. That means killing them now would be nothing more than an unnecessary cruelty, based on a medieval understanding of the law, which is 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 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.