Reveal - The Culprit was........ St. Patrick!
The culprit is St. Patrick. As an old school Christian, he felt that Santa Claus has helped make Christmas too commercial and distracts from its true religious message. He didn’t like that Santa was challenging his position as the only saint in the museum. The X found at the scene of the crime is actually both a cross and a representation of the Greek word for Christ. When people see the word Xmas, they sometimes think that Christ has been taken out, but in actual fact, the X represents the word “Christos”. St. Patrick actually left the X at the scene as a reminder to put the Christian message back into Christmas and to teach Santa a lesson, he had even put the poison in a Coca-Cola bottle, who he identifies with the false commercial Christmas.
If you have solved the mystery, it is because you also understand the true meaning of Christmas and St. Patrick has promised that if enough people solve the case, he will administer the antidote to Santa in time to save Christmas.