After Crypto's new body comes with genitals in the start of the second game, he becomes noticeably less angry all the time. Sure he still takes obscene pleasure in killing humans, but he behaves less what's the word — constipated(?) — and has a much healthier sense of humor, even spending half the time hitting on human women rather than impatiently asking when he can massacre more people.
It is likely that without genitalia, the Furons (including Crypto) were left with a form of sexual frustration that exacerbated their already trigger-happy instincts, preferring to out that repression through violence. With pure Furon DNA becoming more commonplace as the games go on, this could also be the case with the Empire as a whole, revealing that interspecies sex with humans is a popular thing among Furons in Path of the Furon.
