Question: Why did Niander Wallace kill the female replicant right after creating her? I get that he's crazy, but was there any other reason?
Answer: He was being violently petulant at the moment, angry that he couldn't create and control the birth that he just learned occurred with older-model replicants and seeing his new creation as "flawed" by design. Pretty villainous, he cares nothing for the replicants.
Answer: Think of any manufacturing process. Samples of new products are frequently created and then immediately destroyed. Also, the new replicant would require processing, training, etc. It was simpler for him to just dispose of the test.