would you say macaque is something like a necessary evil in the novel?

I would say you are talking about the Six Eared Macaque right? There are a few Macaques in the story. I mean I wouldn't really say he is "necessary" as that isn't really the role he plays within the novel.

When I think "necessary evil" I think more like a force in society that is kept in order to keep the peace or to preserve other social forces that are considered more important to keep than to lose. A necessary evil is an evil that someone believes must be done or accepted because it is necessary to achieve a better outcome—especially because possible alternative courses of action or inaction are expected to be worse.

Which I do not believe the Six-Eared Macaque plays at all.

He is to play the role of the mirror to Sun Wukong. He is supposed to represent the absolute worse of what Wukong is capable of, not really being the opposite of Wukong but rather more of a foil to show Wukong what he could have been (I actually think another great foil character is Erlang Shen for different reasons). A version of Wukong that if he continued to be a warlord, continued to kill without regard for the value of life or consequences that the would turn into him.

The Six-Eared Macaque is not given a backstory so it's hard to say if they have similar backgrounds but rather it is that they are the same species and have the same drive that Wukong had at the start of the novel. Both being of immense power and, like Wukong, The Six Earred Macaque's greatest desire was to get fame and glory and to be recognized for it, despite the consequences.

Personally I believe that the Six-Eared Macaque was inspired by the Buddist saing “The dharma is not to be transmitted to the sixth ear" which means that there isn't a third person around to easdrop, usually between student and teacher. This is only used once when Wukong was speaking with Master Puti and this is where I think that the Six Earred Macaque learned the same skill that Wukong did.

Of course, this also means that he doesn't have a complete understanding. He would always be slightly off with his own teachings, always a step behind, and only be able to imitate what they were eavesdropping on as they were never told directly.

Kind of like how Wukong is supposed to be a master of transformation when he fought Erlang Shen and still lost the SIx Earred Macaque is supposed to discern and have knowledge but he is still just eavesdropping. He is never going to have the real thing.

Wukong is meant to represent Nothingness or emptiness, literally in his name meaning "Monkey Awakened to Emptiness" and how he and the Six Earred Macaque are meant to be of one mind but they are "two mind" that are fighting each other. One being one's natural impulsive thoughts and the other is the self-control people practice to think ahead. They are both inside a person's mind but only one can have full control of the body so to speak. I think a good western example of this would be like Ego vs ID but please don't take that as a one-to-one, it's not!

It is connected to how Wukong is the mind monkey and in order for him to reach his namesake that state of overcoming uncontrollable thought he needs to fight the worst version of himself, which is what I think the Six-Eared Macaque is supposed to represent. This actually goes with how I think Wukong and his connection with Bailong being the Horse of Will show how Wukong has gradually started to overcome his own thoughts and practice self-control though his dedicated willpower. (Bailong is an important allegory yet again.)

Long story short I think that the Six-Eared Macaque is to more represents Wukong's character growth. Rather than be a "necessary evil" he is another milestone to show that Wukong is about to look at the worst traits of himself and through fighting himself and wrestling with who he is, he is able to come to accept himself as well... and by defeating his doppelganger he shows that after battling himself he comes out the victor. Him moving on and changing throughout the journey shows him to be stronger and more sure of himself both mentally and spiritually.

Do you think that if Wukong had not killed the Six-Eared Macaque, the Six-Eared Macaque could possibly have been punished and then tried to be redeemed? I feel that since Heavenly monkeys are one of a kind, it might be one of the reasons why Buddha reacted the way he does after Wukong killed him. It's the way he reacts that makes me wonder what Six-Eared Macaque's fate would have been if it weren't for Wukong killing him.

Although I know that metaphorically, Wukong had to kill the evil part of himself.

That is just it! In the story, metaphorically yeah, Wukong has to kill him. I've talked about this before but Wukong facing off the Six Ears is legit him looking at the worst parts of himself, seeing the murder, the rage, the lack of empathy, everything, and telling himself that he is better than that. The whole ID versus Ego kind of battle of the mind of which one is going to win out and whether Wukong is able to control his murderous thoughts or let them win. But also Wukong looks at the worst side of himself and knows that not only can he overcome it but that he can become a better person. Knowing you are able for great good but also great evil and the need to keep yourself in check else you start going down a dark path. Wukong faces the reality that he is capable of horrid atrocities and needs to hold himself accountable and understand the consequences of his actions.

Wukong had to face his worst inner traits and say "I'm better than that" and he IS! Wukong looks at himself the selfishness, the ruthlessness, the lack of empathy and he hates that about himself but the only way to change it is to confront it head-on and personally stop it. He always trying to do better and that is why it is symbolized in a fight to the death.

But hypothetically, if the Six Ears was NOT a metaphor for facing your inner demons... I think he could be redeemed.

Because Wukong was redeemed as well.

The thing about Xiyouji is that it shows that no one is past the point of recognizing that you need to change and while isn't easy, anyone can self-reflect. Anyone can reach that point of knowing they can change and anyone can be a Buddha... even a monkey.

So with that being my belief of what the story is about I think that perhaps Six Ears could have turned that new leaf as well if given the right teacher and the right motivation. I don't think that Buddha had plans for the Six Ears because he would also know that Wukong would kill him. Even when you know something is going to happen you can still be saddened that it does happen, and I think that is why Buddha expressed his disappointment at Six Ear's death even if it was inevitable.

I don't think the Six Ears was even meant to be redeemed but rather that his needed death was still upsetting to the Buddha considering that Buddhas should not kill.

We see other demons on the journey being taken in as well like the Black Wind Demon, Red Boy, and the Hundred-Eyed Demon Lord. These demons were taken in by heaven and given positions to work under them so this wouldn't be out of the ordinary if given the right circumstances. Two of these demons were redeemed by Guanyin herself and she used the fillets to motivate and highlight their need for self-control. Perhaps If Guanyin didn't use the Prohibitive or the Golden fillets beforehand then it could have been used to monitor the Six Ears.

JTTR did suggest a secret fourth headband Strength for AU ideas so that would be a fantastic suggestion here I would say. He would be either under a buddha or bodhisattva if this was the case (most likely Guanyin since she would have the headband) and he would have to start his own path of self-reflection if he is to even reach enlightenment. Considering he is similar to Wukong then perhaps he would first have to go through a similar punishment under a mountain would be fair but if we compare him to the other demons that were redeemed on the journey it would also be understandable if that was skipped as well.

But those are my thoughts if this was the case as it could be a fun idea!