I've read that あいつ and こいつ can mean "that person over there" or "this person over here", respectively, and also that (at least, as far as I'm aware, for こいつ) they are an impolite way of referring to a person.
In manga I've seen scanlators interpret こいつ in front of another person as "you bastard" or "you bitch"... but also that if referring to a person that they are talking about as あいつ, it gets interpreted as just "that guy" or "that girl".
What scenarios would imply what when using the phrase あいつ and こいつ?