Just like their are warlike Humans, emotional Vulcans and agreeable Tellarites, they are always exceptions and variations to a society. Klingons are no different, Worf's quiet, calm nurse Kaylest was quite peaceful (knife throwing expertise notwithstanding). In short: Klingon aggression is a result of both genes and upbringing. It has been alluded to that Klingons are naturally more aggressive, but it's hard to separate genes from culture. Many Klingons are perfectly able to restrain their impulses and live peacefully.