Is warning people about a person who commits evil a form of slander or backbiting?

Warning
others about a person who does evil and commits major sins is obligatory
(waajib). If a Muslim fears that his brothers may be harmed by those
people’s actions, he must advise them and tell them about these
evildoers, so that no one will fall into their trap. It is also obligatory
to advise the evildoers and those who commit major sins, so that perhaps
Allaah may guide them. The Messenger of Allaah
(peace and blessings
of Allaah be upon him) said: “Religion is sincerity” They said, “To
whom, O Messenger of Allaah?” He said, “To Allaah and to His Book, to
His Messenger, and to the leaders of the Muslims and their common folk.”

(Narrated
by Muslim from the hadeeth of Tameem ibn Aws al-Daari (may Allaah be
pleased with him).