Motive: Jealousy. Spoilers

The motive was jealousy, or at least it was heavily implied. Babtiste was jealous because his girlfriend thought Pierre pleased her better sexually. Nicolas was jealous that Pierre was able to live proud about his bisexuality and resented the fact that Pierre knew Nicolas was not bisexual but gay and didn't want to come to terms with it. Sebastien was jealous because he could never live up to Pierre's image in Lucie's mind. Ultimately they were all jealous of Pierre's confidence and pride which contrasted with Pierre's constant insecurity about his beauty, perfection and life. If you notice Lucie was also concerned about her beauty when she was looking in the mirror. The movie was a bit off in terms of editing and quality, but it's a good one nonetheless. I wouldn't mind seeing an anglo or american remake with a better director without censoring the film's sexuality, nudity and nuances.