Taking Vinaya as a measure of the weight of kamma?

Some Brahmans, strangely those learning Abhidhamma and give into meditation, draw excuses of deeds and weight of kamma from the Vinaya, arguing that a lesser transgression must be lighter kamma. Is such an idea legitimately or just another cherry-picking around to justify personal incapacities and giving favors? What's the different between "retaliation-justice" (in cases one likes to judge god-like) and the Vinaya, the difference between conduct-rules and kamma? Acting according certain community-rules or according the law of nature? *[Note: this is not given to give a stand, nor to trade, exchange or Buddh-ism, but for liberation, gaining rightly unbound.]*