Bonjour Dragana !
The reason faire du mal becomes faire de mal here is because the sentence is negative (nous ne voulons pas).
See also the following lesson:
Du/de la/de l'/des all become de/d' in negative sentences (French Partitive Articles)">Du/de la/de l'/des all become de/d' in negative sentences (French Partitive Articles)">Du/de la/de l'/des all become de/d' in negative sentences (French Partitive Articles)">Du/de la/de l'/des all become de/d' in negative sentences (French Partitive Articles)
I hope that's helpful!
Bonne journée !
