Long back when The Avengers first appeared in the Ultimate Comics universe, they were called The Ultimates. That was an age when people used to be much more relaxed and excited compared to now. Eventually, the Ultimates were discarded, and a new under-cover special ops squad was put together. The new team comprised of Blade, Punisher, and a savage Captain America, all of whom reported to Nick Fury. They preferred going by the name of The Avengers, which was much more suitable to them than the original Avengers.
Since this is a comic book, the Avengers were later segregated into two groups, the New Ultimates and The Avengers. That is when there was a battle between the two squads because that is the only way they could have gone about.
Normally, such street brawls don’t matter much, but, this was a battle which resulted in bodies being dropped. At one point of time, Captain America is battling with Nick Fury on a bridge, and Spider-Man swung by to try and support. Using his Spider-Sense, he jumped on to the Captain to cover him and gets shot by a sniper in the act.
The person who fired the bullet was The Punisher, who gets so shattered by the unintended result of his action that he instantly tries to commit suicide. However, Frank Castle lived to see another day, War Machine destroyed the bridge, and they all went on fighting.
However, things were different with the Spider-Man. He found out that the Green Goblin, who has superpowers in Ultimate Universe, was an invincible monster, and planned to murder his family. He is injured, but, with the Avengers still fighting it out among themselves, he goes on to fight the Goblin and gets killed. The captain is crestfallen, and the Avengers regret the way things turned out to be. That is how a typical comic book crossover turns out to be generally, but, the only difference is that we don’t see a teenager dying in battle.