I am presenting you with an argument of the form reductio ad absurdum, which is a valid form of argument that you will find in any logic textbook.
The following two statements are of the precise same form:
(a) Abortion should be allowed because many of those who are aborted would develop into criminals.
(b) Killing five-year-olds should be allowed because many of those five-year-olds would develop into criminals.
The argument (a) is OFTEN presented in seriousness by those who favor legal abortion.
The argument (b) is of precisely the same form, and is therefore of precisely the same validity as argument (a).
The typical response of those who are unskilled in following logical arguments is:
"I was talking about abortion. I wasn't talking about five-year-olds. Why are you changing the subject?"
The response of those who CAN follow a logical argument is:
"I see that argument (b) is absurd, and therefore, argument (a), being fundamentally identical, must also be absurd."
The argument of the form reductio ad absurdum exposes absurd arguments for what they are. And, it simultaneously exposes those who are unskilled in following, or who have chosen not to follow, because of moral corruption, the logical implications of absurd assertions.
The key word being absurd. Your previous projection of your argument that: "If the fact that abortion kills potential rapists, murderers, molesters, etc., is a justification for abortion, then you have to admit that if we killed all the kindergarteners in America, that would be justified, too, because if we did, we would also be killing lots of potential rapists, murderers, molesters, etc."
Equating the two is absurd.