1.Aang when he lost Appa from the Last Airbender. 

Basically Aang is normally very peaceful and relaxed and a dude who isn't very violent or aggressive and only fights to defend himself and others but after finding out that Appa,his sky bison and best friend,was stolen and kidnapped by Sandbenders..he basically goes down a darker path.

He's more impatient, aggressive(upset)and even kills a Buzz Wasp(he never kills)and it makes sense why..cause he thought he fully lost someone he loves and cares for and is basically the last aspect of his already dying culture.

Thankfully,after seeing a wandering couple give birth to their child Hope,he slowly but definitely starts to regain what he had lost and that leads into him being reunited with Appa.

2.Goku when he went Super Saiyan against Frieza in Dragon Ball Z.

Basically after Frieza killed Krillin right in front of him..Goku,someone who normally Isn't all that ruthless or mean or aggressive ,starts to lose himself in his anger and Saiyan DNA ans being more ruthless and aggressive and intense and only started the end does he calm down and only kills Frieza cause he tried to cheap shot him.

Basically Goku isn't normally someone who torments or wants to break his Enemies but Frieza pushed him.

3.Spider-Man/Peter Parker when he got infected by the Symbiote in Spider-Man/Marvel.

Peter is normally a nice guy and friendly and a good dude and while he does make Quips and such, he's not a particularly ruthless or aggressive fighter but after being taken over by the Venom Symbiote and gaining the black suit,he starts to act not only more aggressive and violent but he also starts to act more aggressive and like a asshole outside of being Spider-Man as well.

He eventually and thankfully takes the suit off when he goes too far and well..the rest is history. 

[Loved trope]When the MC is acting out of character..but it's intentional out of character due to the circumstances and situation.