Three alphas, Deucalion, Kali and Ennis, each of whom were the leaders of their own respective packs, all carried out and indulged in this new trait without remorse. They murdered every member of their own individual packs, subsuming all their collective powers. Afterwards, the three formed the basis of the dastardly and dreaded Alpha Pack. From Deucalion's words, the effect of having killed one's own pack members to take their power seems to have a corrupting drug-like influence.
"I want you to kill one of the them. Do that and I won't have to ask you to kill the others. You'll do it on your own." - Deucalion to Derek