Choice of script in Japanese writing is often a stylistic attempt by a writer to convey some non-standard subliminal meaning. There have been some interesting analyses on this topic done over the years. Just to give you an idea of some of the findings, one study found that certain characteristics were often attributed to script usage, as follows:
Hiragana: feminine, soft, smooth, round, tender, simple, childish, lovely, elegant, etc.
Katakana: novel, foreign, emphasizing, hard, fake, male, futuristic, sharp, jarring, angular, etc.
Kanji: scientific, rigid, masculine, formal, hard, difficult, intellectual, visual, substantial, etc.
It’s clear that 君 is usually written in kanji, so a writer’s decision to use キミ would definitely be an attempt to inject something different into the ‘tone’ of the word. In your example, perhaps it is an extra sense of irritation that the writer is trying to express. Or maybe a sense of urgency. The point is, deciding to use a script which is not conventional is a way to infer different layers of meaning. What that meaning is can depend on things like the writer's intention, the context, the characters, etc.