What if Tobirama didn't Kill Izuna in Naruto?
Before Konoha was established, Tobirama killed Izuna Uchiha that left a huge scar in Madara's heart.

Naruto is considered one of the most popular anime and manga series in the world. We all have a special memory attached to the series. Released in the year 2002, the anime series continued till 2017 and became the most popular in the world.
The series is versatile, to say the least, with its variety of characters, clans of shinobi, plots twists, backstories, tragedies, and so on. The series introduced us to many legendary shinobi and their Jutsu.

Tobirama Senju, the Second Hokage of Konoha was one of those shinobi who dedicated his life and talents unconditionally to the village. As a brother, he was an excellent advisor to the Hokage. As a Hokage, he was just as amazing a leader as his older brother.
And, as a shinobi, he possessed talents and strength unparalleled to anyone else. Like Hashirama and Madara, Tobirama also played a major role in forming the village alongside them.
However, like others, he was still a slave of the rivalry among the Senju and Uchiha that had been going on for centuries. Because he killed Izuna Uchiha, the younger brother of Madara, the conflict weighed on the latter's mind forever.
Madara only stayed in the village out of his respect for Hashirama, but he couldn't let go of the death of his only remaining sibling. This gives rise to the question of what could've happened if Tobirama had never killed Izuna.

The Rivalry Between Senju and Uchiha

Ever since the beginning of Ninjutsu, the Senju and Uchiha clans have been enemies. This is because of the feud between Asura and Indra that was passed to their descendants as well.
As the two most powerful clans in the world, they were constantly at odds with each other. The Uchihas were extremely strong and possessed several times more chakra than any member of a normal clan.
However, in terms of strength, only the Senjus were any match to them. This led to a never-ending conflict among clans with no clear victor whatsoever. The rivalry finally ended after Hashirama and Madara made an alliance and established Konoha as the first Shinobi village.

What if Tobirama didn't Kill Izuna?
Izuna's death was a huge blow to Madara. Tobirama only killed him as a consequence of the war among clans without any personal feelings. Tobirama and Izuna were having a battle in the midst of the conflict and both were having swords in their hands. So, Tobirama didn't play tricks to kill him, rather it was more of a choice he had to make for his clan.
1. Konoha May not Have been Established

Just as how Tobirama hated the Uchiha, Izuna felt the same way about the Senju. He never wanted his clan to get along with Senju and would've stopped Madara from doing the same.
Madara genuinely cared about Izuna and would've given up after continuous pursuit.
2. Madara's Powers would've been the Same

The reason Madara was so absurdly powerful was that he didn't have any limitations to his Sharingan. As we know, Mangekyou Sharingan takes a serious toll on the user's chakra or eyes, just as what we saw in Itachi and Kakashi.
However, due to Izuna's death, Madara was able to take his eyes for himself and was granted the powers of the Eternal Mangekyou Sharingan. This was also a major factor in Madara awakening his Rinnegan.
3. No Fourth Great Shinobi War

It is true that Izuna's influence on Madara could've stopped him from having a part in the establishment of Konoha. However, if he had been alive, Madara wouldn't have been so easily manipulated by Black Zetsu. This way, he wouldn't have had to use Obito and start the Fourth Great Shinobi War.

Thanks for reading!!