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.]
You must be dissatisfied with my quote (in this answer) of killing humans being a parajika offense for monks vs. killing animals which is only a pacittiya offense. A parajika offense is met with immediate and irreversible dismissal from the monastic order. A pacittiya offense is met with confession, usually with the intention to not repeat it.

Firstly, let's get this right:


Killing is against the first of the five precepts
Business in meat is against Right Livelihood.
Keeping the five precepts is one of the criteria for stream entry according to AN 10.92. 


However, does this mean killing any living creature has the same consequence or same effects? Does killing a mosquito bear the same consequence as killing an arahant?

The answer is NO.

According to AN 6.87, there is actually a list of people a person could kill or injure, and such a person will never be able to have the right mental state or moral capacity to learn the Dhamma:


  "Endowed with these six qualities, a person is incapable of alighting
  on the lawfulness, the rightness of skillful mental qualities even
  when listening to the true Dhamma. Which six?
  
  "He has killed his mother; he has killed his father; he has killed an
  arahant; he has, with corrupt intent, caused the blood of a Tathagata
  to flow; he has caused a split in the Sangha; or he is a person of
  dull discernment, slow &amp; dull-witted.
  
  "Endowed with these six qualities, a person is capable of alighting on
  the lawfulness, the rightness of skillful mental qualities even while
  listening to the true Dhamma. Which six?
  
  "He has not killed his mother; he has not killed his father; he has
  not killed an arahant; he has not, with corrupt intent, caused the
  blood of a Tathagata to flow; he has not caused a split in the Sangha;
  and he is a discerning person, not slow or dull-witted.


According to MN 86, Angulimala killed many human beings, but he could still change and become an arahant. So, probably, he did not commit the transgressions above (killing father, killing mother, killing arahant, injuring Buddha):


  And at that time in King Pasenadi's realm there was a bandit named
  Angulimala: brutal, bloody-handed, devoted to killing &amp; slaying,
  showing no mercy to living beings. He turned villages into
  non-villages, towns into non-towns, settled countryside into unsettled
  countryside. Having repeatedly killed human beings, he wore a garland
  (mala) made of fingers (anguli).
  
  (After the Buddha transformed Angulimala)
  
  Then Ven. Angulimala, dwelling alone, secluded, heedful, ardent, &amp;
  resolute, in no long time reached &amp; remained in the supreme goal of
  the holy life for which clansmen rightly go forth from home into
  homelessness, knowing &amp; realizing it for himself in the here &amp; now. He
  knew: "Birth is ended, the holy life fulfilled, the task done. There
  is nothing further for the sake of this world." And thus Ven.
  Angulimala became another one of the arahants.


On the other hand, according to DN 2, King Ajatasattu was unable to effectively learn the Dhamma because he had arranged to kill his father in order to become king:


  So King Ajatasattu, delighting and rejoicing in the Blessed One's
  words, rose from his seat, bowed down to him, and — after
  circumambulating him — left. Not long after King Ajatasattu had left,
  the Blessed One addressed the monks: "The king is wounded, monks. The
  king is incapacitated. Had he not killed his father — that righteous
  man, that righteous king — the dustless, stainless Dhamma eye would
  have arisen to him as he sat in this very seat."