Sadhu means excellent. By following His advice. Sacitta pariyodapanam Etam Buddhana sasanam. Not to do evil, To do good, To purify one's mind, This is the advice of all the Buddhas. Papa or Akusala is the Pali word for evil.
Evil is that which soils our mind.
Because it causes suffering to oneself and others. The roots of evil are lust, hatred and ignorance. Lobha, dosa and moha. They are caused by deeds, words, and thoughts. They are caused by deeds, words, and thoughts.