1) The Mishnah (37a) states that according to Rebbi Yehudah it is possible that an animal is only Mu'ad for Shabbos. Rashi explains that since the animal does not work on Shabbos he is liable to become arrogant. This suggests that the animal has similar habits that some humans have when they are on vacation.

2) There is a Gemara which states that an animal posseses a Yetzer ha'Ra. This is in Berachos 61a (10 lines from the top of the page), where the Gemara says that the fact that animals damage, bite, and kick shows that they possess a Yetzer ha'Ra.

3) The Gemara in Berachos 33a states that oxen are dangerous in the month of Nisan. Rashi (DH b'Yomei) explains that when the ox sees the fields full of grass he becomes arrogant and the Yetzer ha'Ra enters it. This is similar to the Rashi we saw above from Bava Kama 37a, but in Berachos 33a Rashi adds that the animal possesses a Yetzer ha'Ra.

4) However, the evil impulse of the animal may not be the reason for why the animal is killed if it kills a person. The Ramban, to Bereshis 9:5, writes that since an animal does not possess Da'as, it is not logical that it should receive punishment or reward. The Ramban to Bereshis 8:1 also writes that when it says that Hash-m remembered Noach and all the wild and domestic animals, he did not remember the animals because of their merits. Rather, he remembered what He had promised that the world, with the species of animals which He had created, should continue to exist.

5) The Ramban there (9:5) writes that it is a "Gezeiras Melech" that any animal that kills a human must be killed. It is a decree of the King of the World that the goring animal must die, but according to the Ramban it is not a punishment.

Kol Tuv,

Dovid Bloom