Why did he kill the newborn replicant?

I'm not a huge Jared Leto fan and I thought his dialog was a little too much. It was what the character called for but it still seemed forced as of they tried to make him cooler than what he was.

But to answer your question, he kills her because she lacked the ability to ovulate. If you follow his dialog, he refers to this as he touches her stomach right before he stabs her.