Peter's part
First of all, you have to know that Peter was, as some people said, a bit eccentric. He loved animals as much as he loved people, maybe even more than people: he wouldn’t even hurt an annoying fly.
But let’s stay to the subject: the, apparently, dead unicorn. Someone else would have turned back and simply walked away, someone else would have called the police or would have … whatever. Peter did none of these things: He started crying.
A unicorn is a wonderfull creature indeed: It is entirely white, with long silver manes and tail, and beautifull spark blue eyes. Peter loved animals, and the sight of a dead miracle makes that I understand his tears. But what followed, I always found hard to understand why he did it.
While trying to consolate the foal by giving it sugar lumps, he decided to hunt the murderer down, and finish him. To be short (I don’t want to describe it very much, because I find it horrible), he found the trace of the murderer, followed it, and killed the murderer by strangling him.