After attempting to make sense of the entire issue myself, I can no longer decide what I believe in the topic here. Between religious alliances and political alliances this appears to be a complex situation on all ends. Why would Catherine was Coligny dead after marrying her daughter to a Huguenot king, and then sanctioning one of her opponents to lead a massacre of Hugenots?
A lot to unravel here. The idea that Catherine de Medicis was central in the massacre doesn't have as much support as it once had (Arlette Jouanna, in her "La Saint-Barthélemy. Les mystères d'un crime d'état" doesn't support it, for instance), and is a variation on the machiavelian queen trope that has to be questioned.
De Guise was put in charge of the massacre mainly because he was one of the main figures of the anti-Huguenot side of the wars of religion: the Guise family could even be said to have started the wars, since they committed the massacre of Vassy in 1562 which set the whole thing off. He didn't agree with the Queen, because his whole family's aim was to gain power (thus the marriage of Mary Stuart with François II a few years prior), and this formed the basis of their opposition. In being anti-protestant however, they were on the same side.
The real mystery of those days is what made the King change his mind between the day of the attempted murder of Coligny (he promised justice for him and sent his own doctor to look after him), and the following day when he ordered the execution of the protestants leaders. The decision was taken during a late night council meeting, and the role of everyone in it is unclear as no records were kept of such meetings.
Latest and best book for this is Nicolas Le Roux, Les guerres de religion, Paris: Belin, 2014.