The Gemara in Sanhedrin (71b) teaches:

The rebellious son is judged based on his future...The Torah foresaw his future ... In the end he will exhaust his father's assets. He will seek his accustomed habit and will be unable [to afford it], so he will go out to the crossroads and hold up people. The Torah says: Let him die "innocent" and let him not die "guilty."

At first glance, Chazal's view that the rebellious son is judged based on his future contradicts their comment in Parshat Va'era regarding Yishmael and Hagar, where it says: "For G-d has heeded the cry of the youth in his present state." (Bereishit 21:17) Chazal comment that he is judged according to the actions that he does now, and not based on what he is destined to do. Rashi writes (based on the Gemara R.H. 16b and Bereishit Rabbah):

The angels were saying accusingly: Master of the Universe: One whose descendents are destined to kill your children with thirst – you draw up a well for him?He answered: Now what is he, righteous or wicked?They said to him: Righteous.He said to them: Based on his current actions I judge him.This is what is says, "in his present state."

These passages contradict one another, since the rebellious son is judged based on his future, whereas regarding Yishmael we see that G-d's policy is to judge the person based on his current actions and not based on the future?

We can resolve this contradiction based on a halachic principle found in the Responsa of the Chatam Sofer (Y.D. #256). The Shulchan Aruch (O.C. 32:3) writes that tefilin must be written with black ink. This law is an oral tradition to Moshe, and is alluded to in the pasuk in Yirmiya, "I write on the scroll with ink." If a Sefer Torah is written with black ink, and after a few years the writing becomes dull and turns red, the Sefer Torah is still valid. However, if it is written with ink whose nature is to turn red – even though now it is black, the Sefer Torah is invalid. This is not considered black, since it has a defect that it inherently turns red, just as in the law of treifot we say, "Whatever is destined to become punctured is considered as punctured [already]." This means that an animal that has a disease, such as a lung that has a blemish and stands to become punctured – we consider it non-kosher already now, since the hole is certain to come of its own accord. I.e., anything that is expected to change of its own accord, without man's activity, we look at the future state as if is exists already.

This is the difference between the rebellious son and Yishmael. Regarding the rebellious son, whose future the Torah foresaw, the defect began already now. The Torah sees that his current fault is going to spread more and more until he stands on the crossroads etc. In this case, we judge him based on his future, like a treifah. However, Yishmael was innocent at that time, and G-d knew that the fact that his descendents would kill Israel with thirst is not a result of Yishmael himself according to his current behavior. Therefore, he is judged "in his present state."

It is possible that this is the reason that we read the portion of Hagar and Yishmael on Rosh Hashana. When we come on the Day of Judgment and say to G-d that we regret the past and accept that we will not return to this sin in the future – G-d knows the future, and knows that we will sin once again, so how can he accept our repentance? Therefore we ask that G-d judge us, "in our present state," according to our state at the moment. Since Israel inherently are innocent, and in their essence they all want to do the will of their Master, their sins are only external incidents. If, G-d forbid, they will sin in the future – this is not due to their present fault, but a new incident. Therefore, we read this portion of Rosh Hashana, so that G-d will judge us based on our actions at that time: "in his present state."