Because he was the perfect fit.

Simple misunderstanding

Micheal says that "they didn't recognize me at first, but then they thought I was you."

This would possibly imply that Ennard thought that Micheal was William, and ended up killing an innocent man.

And why kill William? Who wouldn't?

what gabrial said

Because, they thought michael was william. thus they wanted to exact revenge on him

Elizabeth was in control of Ennard at the time of Sister Location Location, Elizabeth wanted Ennard wanted Michael because he was the only one who can help them Escape.

Besides, I no longer believe that Circus Baby even lied to any of the animatronics, saying that Michael was William. She reassured Ballora that he was actually going to help them, not hurt them..

I don't think the kids possessing The funtimes Know who William is Except for Elizabeth but She didn't know What William did at the time.

and the reason why they want to kill you is because They want to leave.