Why did Leyla not become evil?

Overall, it seems as if "Immortal" is synonymous with evil. All of the original seven immortals are varying degrees of evil, from undiluted, sneering evil (the majority of them) to evil with occasional flashes of petty maliciousness or even love for individual people (Vizier and Faysal). More obviously, when some of the Immortals’ plans come to fruition and a significant percentage of the population of Istanbul joins their ranks, they seem to all become fairly callous, even toward other Immortals. This phenomenon is probably best illustrated by Zeynep, who seems to retain her memories and much of her personality, but says that the things that used to matter to her (such as compassion or her former friendships) no longer seemed of much importance.

All well and good, quite consistent. Except, what about Leyla? When she was made into an Immortal, her personality did not seem to change significantly. Whichever Immortal had created could control her directly, making her do all sort of unpleasant things, but she herself did not become evil. Why did Leyla not become evil when she was made an Immortal, even though both the original articles and the ones of the vaccine were pretty unpleasant characters?