Experimentation is too dangerous. I understand this can lead us to a better solution or even a cure to save those who were recently infected by the taint. Unfortunately, the Architect isn't interested in finding a better solution nor a cure. His only interests is in himself and fellow darkspawn like him. An there's no guarantee he'll keep his word. Its better to get rid of him without worrying about it biting us in the arse.
It does not matter what his interests are, only the results.
What the Architect uncovers about the Taint can benefit others, even if the intent is only to help his kind.
And you're right that the experiments have also been hazardous to others, non-Darkspawn, but any effort made towards the Darkspawn issue is going to have that danger involved. At status-quo, we have Blights, which threatens Thedas in its entirety. Is it better to do nothing and just allow them to take place, or make an effort -- dangerous though it may be -- to lower their resulting body counts of this existing problem?
And since the Architect "awakens" Darkspawn by severing their connection to the Old Gods, what happens after the last Archdemon is killed come Blight VII? Unlikely the darkspawn and their broodmothers are all just going to magically disappear...