Animals O.K. to Kill

Hafsah (radi Allahu anha), the wife of the Prophet (sal Allahu alaihi wa sallam) said: "The Messenger of Allah said, 'There are five animals for which there is no sin on the one who kills them: Scorpions, crows, kites, mice, and vicious dogs.'" [Sunan An-Nasai - graded sahih]

In addition, the Prophet (sal Allahu alaihi wa sallam) ordered the killing of geckos. Stepping on ants for no reason, killing or hurting those animals from which one receives no harm is sinful. But the above mentioned are animals from whose harm we have been told to protect ourselves.