In his search for immortality, Lucretia's Master experimented with the rejuvenating qualities of blood which, he suspected, was the reason vampire bats live a surprisingly long life on their one food source.
After squeezing out a number of the unfortunate critters of their life juice, he just needed to test his theory out.
Had he taken into account that his little victims may have feasted on a sleeping lycan at some point, Lucretia may not have turned into the 7'5", blood and soul draining nightmare that she became.