Considering how many shinobi are perturbingly bloodthirsty, the latter might be a problem. Throughout the series' 700 episode run, dozens of ninja have proven so violent that it seems like their primary ethos in life. By identifying them, it becomes easier to gauge which shinobi to watch out for and what they have already done so far.
Considering that Pain's goal is specifically to make other people miserable, it comes as little surprise that violence is his method of choice. The only reason he collected Tailed Beasts was to destroy the Five Nations and unite the world under mutual suffering. He hoped that their anguish would serve as the foundation for true peace.
During his arc, Pain was stunningly successful. Not only did he murder Jiraiya, he also destroyed most of Konoha with a single attack. Even Naruto would not have been able to stand against him without extensive prep time and Hinata at his side.