There are loved ones in the glory
Whose dear forms you often miss.
When you close your earthly story,
Will you join them in their bliss?
Will the circle be unbroken
By and by, by and by?
Is a better home awaiting
In the sky, in the sky?
In the joyous days of childhood
Oft they told of wondrous love
Pointed to the dying Saviour;
Now they dwell with Him above.
You remember songs of heaven
Which you sang with childish voice.
Do you love the hymns they taught you,
Or are songs of earth your choice?
You can picture happy gath’rings
Round the fireside long ago,
And you think of tearful partings
When they left you here below.
One by one their seats were emptied.
One by one they went away.
Now the family is parted.
Will it be complete one day?
– A. Habershon, C. Gabriel
Ken Levine, the creative director of Bioshock Infinite, stated in an interview (5:47) that it’s the lighthouse keeper.
The lighthouse serves as an entrance to Columbia, and as we see at the beginning of the game, used by pilgrims and newcomers that have been accepted into the city. Such a place would have a keeper, a guardian. There are religious motifs all over the place, because as any good Columbian, he’d be a believer. There are charts in the lighthouse that show where the city is going to be depending on the time of the year, and the pattern it follows.
And to activate the chair that ejects you into the city, you need a password, the bell combination, that only said lighthouse guardian would know. Now, in the Lutece labs, you find a picture of the lighthouse with a red circle marked around it and the words “ONLY ONE OBSTACLE” next to it. Their plan was to send Booker through the lighthouse, and the only obstacle was the man guarding it himself.
So that’s it. The Lutece have shown to be quite cold in their behavior and not care much about other people’s lives when they get between them and their experiments. Maybe they didn’t do the deed themselves, they might’ve just hired another nameless thug to do the dirty job.
But basically: It’s nobody but the Lighthouse keeper, who’s been tortured and murdered in order to get the means for Booker to get into the city. They might’ve left the “Don’t disappoint us” message to add color or feed him the idea that he’s working for some shady criminal organization.