But he knew he'd be a vampire in 1897... I'm guessing he did. If he hadn't made that difficult choice the series wouldn't be the interesting thing that it is, but he had to know he'd end up killing people.
Not to mention that with the writers preoccupation of reincarnation through the series, they could've had Sharon Smith come back and play a teenage girl who came to live at Collinwood, or maybe someone who befriends Jamison, or maybe even as Kitty Soams' daughter or niece or cousin, and then instead of having Kitty be the reincarnation of Josette, they could've had Sharon's new character be the reincarnation of Sarah. I think that would've been even more gripping if the Leviathans had've taken Sarah's reincarnated self hostage instead of Josette, because Josette was constantly in danger, whereas Sarah was only in danger once in 1795.
But then she wouldn't appear in 1967 as a ghost of a child from 1795. If anything, her ghost would be of her last reincarnation, wouldn't it?