Should one search for malicious websites and report them to the authorities, or is it sufficient to report only what one encounters?

Do not seek out objectionable websites unless you are an official specialized in combating such matters. What you encounter by chance is sufficient, for one is not commanded to seek them out, lest it lead to falling into sin. Allah Almighty says: "O you who have believed, do not follow the footsteps of Satan. And whoever follows the footsteps of Satan - indeed, he enjoins immorality and wrongdoing." It is better to suggest to the authorities means of resisting corruption and entrust the task to a specialized committee. You will be rewarded for your enjoining of good and forbidding of evil.