Question for those past fourth part of Bioshock

[kkcountry]

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Spoiler related to killing him: <font color="white"> you will eventually gain access a special/small area in a later level if you hold off on killing him. Gaining access to this area also gives an achievement. </font>

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<font color="white">where is this again? i barely stepped inside the door and then got sidetracked and forgot to come back</font>