Okay, so I think I understand why it tends to bother me that Hiruzen Sarutobi is so widely hated while both Naruto and Hashirama seem so unanimously loved (by many of the same people), and I’m going to try to explain:
(Disclaimer: a) I 100% get why people hate Hiruzen and don’t think it’s wrong, and b) I don’t think people should hate Naruto and Hashirama either. I love all three of them and think all three have flaws)
There is a recurring character archetype in the series of the uncompromising idealist. This character is highly optimistic and sees the best in people, often overlooking people’s flaws, especially those of the ones they love. They have strong ideals and powerful, undefeatable hope; they will hold out for what is right, and not settle for less. They are very reluctant to acknowledge a situation in which their ideals are not attainable, and they struggle to take a different path according to that reality.
This is an archetype that I think Naruto, Hashirama, and Hiruzen fit into.
Because the series has several embodiments of this archetype, it explores it, and how it plays out, in a few very different ways.
With Naruto, his undying hope and drive to find a solution that didn’t involve anyone being hurt, and his refusal to give up on his friend, turned out to be helpful. Nagato changed his mind, Obito changed his mind, Sasuke came back, people were able to let go of their pain and hatred. It all worked out.
With Hashirama, results were kind of mixed. By striving relentlessly towards his ideal vision, he succeeded in unifying the clans of the village, but Madara (and much of the Uchiha) was still disillusioned. Because of his refusal to give up on his friend, Madara was able to hurt many people — Hashirama could’ve defeated Madara if he was willing to give up on him and kill him, but he wasn’t ready to let go of the idea that he could change him — but ultimately, his commitment to his vision of the village was able to give him the resolve to defeat Madara for the good of the village. Hashirama’s idealism had good and bad results.
Hiruzen is an example of the failings of this archetype. His story illustrates how it looks when things play out badly, and those same traits are a fatal weakness. Many times, his optimism and idealism made him hold onto unrealistic hopes for too long and put off difficult or unpleasant decisions — an inaction that allowed these decisions to fall into the hands of Danzō. He didn’t want to believe that diplomacy might not be able to fix the situation with the brewing Uchiha coup, and he didn’t find a solution fast enough, allowing Danzō to take his own (evil) course of action; he didn’t want to acknowledge the malevolence in Orochimaru and let go of his high hopes for his potential until he had already hurt many people, and even then he couldn’t bring himself to do what needed to be done and kill him; he turned a blind eye to Danzō’s crimes because a) he couldn’t view his friend as a bad person, and b) was somewhat aware of his own faults and relied on Danzō’s ruthlessness to make up for his own softness and hesitation. Time and again, Hiruzen’s uncompromising idealism, optimism, and hope led him to make the wrong decisions and cause (or at least allow) much harm.
So, as different embodiments of this same archetype: Naruto illustrates its potential for great good; Hashirama illustrates its potential for mixed results; and Hiruzen illustrates its potential for great failure and harm.
Hiruzen fucked up real bad. He didn’t want to / couldn’t deal with difficult realities or with seeing evil in people he loved, and his inaction allowed terrible things like the Uchiha Massacre, Danzō’s crimes inside and outside the village, and Orochimaru’s experimentation to take place. Many people were hurt because he failed to do his job. There are many reasons to hate him.
But I think that when it comes right down to it, the main difference between Hiruzen and characters like Naruto or Hashirama is the circumstances. The three characters have similar strengths and weaknesses, but there were different people around to take advantage of them.
It is also worth mentioning that Hiruzen was aware of his failings; he felt great guilt and shame over his mistakes, and he ultimately died doing what he could not before in order to protect the village and its people. It was very, very late, but at least in his last actions he did what needed to be done. This doesn’t make up for his failings, but it makes it a lot more difficult for me to hate him.
Anyway those are my Thoughts