Well, the goat is quite an evil individual. We know this. I didn't consider poisoning to be the cause, because that would mean that someone close to Matt (most likely a female) was poisoning him on a regular basis, even in the hospital. Claire and Isidore are good women, and they would never do such things, and Matt's mummy loves him deep down. As for Danfield's mother, she's a serpent, but her husbands career isn't in complete ruin, so she has no reason to do that, and just like Matt's mummy she is not close enough physically. The only other thought would have been a woman from school, and the only girl enemy left, so there weren't any suspects for such a crime.