“I think there is a statistic that shows all serial killers started their careers by torturing and murdering animals.”
Not quite. There was a theory proposed back in the 60s that this was one of three behaviors (the other two being bedwetting and pyromania) that could be indicators of future violent/sociopathic behavior. That theory has not held up well to scrutiny over the years, but it is embedded in the popular imagination so people still think it is true.
http://listverse.com/2013/01/02/10-most-common-traits-of-potential-serial-killers/
http://www.psych2go.net/90-of-serial-killers-admit-to-killing-animals-before-killing-humans/
Respectfully, I beg to differ.