Interestingly, the episode started off as seen through the eyes of an unreliable narrator, which disingenuously set Alpha up as a stress-worn mother driven to the brink by an impossibly desperate situation.
Whatever the truth of her distant path is, Alpha was almost definitely a dark-minded and domineering individual long before the walkers got to shambling.
The TV show, however, made efforts to give Alpha more dimensions, which is clearly the smart way to go with someone as awesome as Samantha Morton in the role.