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 or d' in negative sentences (partitive articles)">Du, de la, de l', des all become de or d' in negative sentences (partitive articles)">Du, de la, de l', des all become de or d' in negative sentences (partitive articles)">Du, de la, de l', des all become de or d' in negative sentences (partitive articles)

I hope that's helpful!
Bonne journée !