Near the film's climax, Rocky gets chained to the area where the girl named Cindy had been kept. It had already been revealed that Cindy killed The Blind Man's daughter, and as a result, he impregnated Cindy to replace his child, with the intention of setting her free when she gave birth. When Rocky, Alex and Money invade his house, The Blind Man, not being able to see, accidentally kills Cindy, and blames Rocky, telling her "she must be held accountable". He then prepares a semen sample, cuts Rocky's pants and prepares to inseminate her the way he did to Cindy. However, before he does so, he tells her he is not a rapist and that he never forced himself on Cindy.
There are two explanations as to why he chose to do it this way. 1.) He does not want to get charged with the crime of rape. If he decides to turn Rocky free when she gives birth, she could easily claim she was raped by him, and he could be charged if they found the evidence. The same could have happened when Cindy gave birth. Whether this is a realistic motivation is unclear, because even though the Blind Man can claim self-defense if he was questioned about the death of Money, Cindy and Alex, he will nevertheless be guilty of kidnapping Cindy/Rocky, holding them against their will and forcefully inseminating them. Even if a jury believes the insemination story, it will most likely earn him a significant prison sentence, and he will not be able to care for his new child in prison. 2.) While he is crazy and wants another daughter so bad, he does have a conscience and a twisted sense of morality. He believes that Cindy and Rocky simply owe him a child, so he will not go to an unnecessary extreme to get one. He even promises to let them go afterwards, but it is questionable if he would keep this promise, since this would almost certainly lead to prison time for him (as described in 1). While one may argue he is going quite far in forcing someone to carry his child, he mostly wants them to understand what pain is and to see how he feels. As for the killings, he was defending himself and his house when the trio broke in, and not just killing in cold blood.