Honestly, when they said it’s gonna be scary, I though it was gonna be just a few deaths in the first episode, not a crap ton of mutilated children. I don’t think eleven did it.

I agree with The345bob - i think we were supposed to believe that the "number" who committed the massacre was Eleven - but I don't think it was her.

In particular, note that Brenner said "what have YOU done?" thus avoiding identifying the child by number.

I think the murders may have been committed by 001.

I also think that the fact that we got a camera shot of a door labeled 013 means that 013 will be making an appearance this season.