In fanfiction, or any writing really, when characters take a tone of voice that one would image to be outraged. The author therefore if forced to not only conclude the character's outburst with an exclamation point (or several) but also use caplock to make sure the character's fit is fittish enough.
In this scene, in which Harry is talking to some adult figure, most likely Remus Lupin or Professor McGonagall, he is being quite capslocky:
"Harry, I know you are upset about the Headmaster..."
"YOU DON'T KNOW ANYTHING!"
"Harry, honestly, you really--"
"I DON'T UNDERSTAND WHY YOU WON'T LEAVE ALONE TO WALLOW IN SELF-PITY BECAUSE ALL OF MY LOVED ONES ARE DEAD!!!!1 YOU DON'T KNOW ME!! DON'T EVEN TRY!"
"DON'T HARRY ME!"
"Fine." ::walks away::
::Sobs:: "No one knows me anymore." ::Is depressed::