We don't know why the speaker is angry with their friend for the simple reason that they don't tell us. In all likelihood, though, it was probably over something incredibly trivial (as most arguments are, especially those between close friends). Perhaps they were arguing over money? Or politics? Or maybe they were romantic rivals? The truth is we just don't know.

But one thing we do know is that the speaker felt able to express their feelings toward their friend, thus assuaging their anger. This would appear to confirm our initial impression that the argument between the two friends was over something relatively unimportant.

The same, however, cannot be said of the speaker's anger towards their foe. Instead of expressing their terrible wrath, they let it eat away at them from the inside until the only way that they can make their foe aware of how they really feels is by killing this hated person.