I guess something evil is going to be done in the future, but why is present perfect ("you've done") used here instead of future perfect ("you will have done")?
Present perfect tense is used here because when or whether he will "do something evil" in the future is unknown. Future perfect tense (you will have done) is used to describe doing something in the future that will be completed by a certain time. Here are some examples of "you've done" and "you will have done": Ex: Call me when you've done the paperwork. (Because it is unknown when the paperwork will be completed, "you've done" is used) Ex: She will have done the paperwork by Tuesday, so we can look at it on Wednesday. ("Will have done" is used here because there is a definite time when the paperwork will be done)
Get more Native English Expressions >