In this anime-esque example, which is correct?:
And why? Because I’ve heard it used with number • 2 • を (wo), but other verbs seem to always use が, for example: 「私はエミリアが好きです」 Is 好き a noun in this context?
Both sentences are perfectly correct, but the meanings are totally different. Let's start with 殺す because it's a rather simple transitive verb which takes one subject and one object. Conclusion first, those two sentences translate to:
- 私はあなたが殺す = You will kill me. / It's you who will kill me.
- 私はあなたを殺す = I will kill you.
In both sentences, 私 is marked with は. But Sentence 1 says "you kill me" and Sentence 2 says "I kill you". Why? Let's review the basic functions of は, が and を.
There are many exceptions, but these should be enough as a starter. In Sentence 1, the subject is clearly あなた because it's unequivocally marked with が. So 私 will be interpreted as the object of 殺す. In Sentence 2, the object is あなた because it's unequivocally marked with を. 私 will be interpreted as the subject of 殺す.
Regarding 私はエミリアが好きです, there is no verb in the first place because 好き is something called a na-adjective. For details, see: 私は猫が好き and 猫は私が好き