In the Night 6 call, The phone guy sounds all depressed. Worried, actually. He asks what you are still doing at Freddy's, since the place closed down already. He talks about how the Yellow suit (Golden Freddy) was taken over, and someone killed children with it. Apparently, the animatronics hate rapists, child molestors, MURDERERS, etc. The animatronics go after Jeremy, the guy you play as. He is the only person that has access to the animatronics. In conclusion, Jeremy is the murderer. Not Mike from the 1st game.
How would Jeremy be the murderer if Mike was there first? He wasn't there first. Jeremy's paycheck is dated back to 1987. He was fired partly because of the Bite of 87. Foxy was not one of the animatronics during that time, but Mangle and Freddy were. Referring back to the call, he says at the end of it "Just finish your night. When the place opens back, I will take the night shift". Meaning that Mike was the Phone Guy.
In conclusion, FNAF2 is a prequel, Mike is the phone guy in FNAF2, Jeremy is the murderer, Jeremy was fired in 1987, most likely BECAUSE of the Bite of 87', Mangle or Freddy caused the Bite of 87.
It is unknown why Freddy, Chica, Bonnie, Foxy, and Golden Freddy are there, maybe as replacements if animatronics tend to tear up, or something? That's just a theory of mine, probably wrong and not confirmed.