Is there an achievement to not kill any of the killable people? I'm playin through it again on hard, and I got up to the bit where you confront Stanley Toole. He's about to give this big monologue about how you totally shouldn't shoot him, then a hypnotized splicer ran in behind me, shot him, and shouted IT'S JUST YOU N ME AGAINST THE WORLD!!
 
Gggaaarrrggghhhhhhh!!!

Woah! Another thing: I just killed a Big Daddy, and when I went to search him, his name was Mark Meltzer.

" Woah! Another thing: I just killed a Big Daddy, and when I went to search him, his name was Mark Meltzer. "

" @I_smell said:

" Woah! Another thing: I just killed a Big Daddy, and when I went to search him, his name was Mark Meltzer. "

If I recall its not the big daddy. Its a cat named that. Was it in that frozen room? Also yes. There is a achievement to not kill any of the key people.  "

" @The_Laughing_Man said:

" @I_smell said:

" Woah! Another thing: I just killed a Big Daddy, and when I went to search him, his name was Mark Meltzer. "

If I recall its not the big daddy. Its a cat named that. Was it in that frozen room? Also yes. There is a achievement to not kill any of the key people.  "

No, the cat has a weird German name. Mark Meltzer was an audio log character that was turned into a Big Daddy.  What level did you find him? "

" @natetodamax said:

" @The_Laughing_Man said:

" @I_smell said:

" Woah! Another thing: I just killed a Big Daddy, and when I went to search him, his name was Mark Meltzer. "

If I recall its not the big daddy. Its a cat named that. Was it in that frozen room? Also yes. There is a achievement to not kill any of the key people.  "

No, the cat has a weird German name. Mark Meltzer was an audio log character that was turned into a Big Daddy.  What level did you find him? "

Ya the cat had a name from some theorist right?  "

I don't know about the cat-theorist, but I do know about that Big Daddy you're talking about:
 

... 
 
 
LOL

The cat is Schrodinger's Cat, named after a theory in quantum physics.
 
Mark Meltzer is someone who leaves a lot of audio logs through the game, like End_Boss just said. He's also the centre of a huge viral campaign crafted by 2K before the game came out. There's a bunch of stuff on the internet that follows how Mark Meltzer discovered the big sister, then lost his wife and daughter. He devoted his life to following clues and finding Rapture. The story ends when he finds the lighthouse.
Then the story picks up again from the audio logs in Bioshock 2; and I guess it ends with you killing him and either killing or saving his daughter, whether you know who they are or not.
 
It's on Gil Alexander's level btw. He's just a totally regular big daddy, apart from the name.