The baby (spoilers)
The baby was, quite obviously to me, the devil himself.
We get a hint of that from the text written on the wall behind the painting that falls down: “ And no wonder, for Satan himself masquerades as an angel of light.”
The baby is referred to as a “miracle” by everyone in the convent throughout the film. But I suppose the point is that it was quite the opposite: an evil, diabolical being.
This is why she had no hesitations in killing it.