Romeo and Juliet: Act 3, Scene 4

It's generally believed that Romeo and Juliet could easily be a comedy IF ONLY Mercutio and Tybalt hadn't died, and that their duel is what turns the entire play from a comedy into a tragedy. However, it can effectively be argued that this is the scene that really removes any hope of a happy ending. And it's ALL CAPULET'S FAULT.

Side note: People today take months, even YEARS to plan a wedding, but the Capulets are just like... "three days, it's all good, let's do this". Granted, this is partially because they don't want to have a big lavish celebration in the wake of Tybalt's death, but still. THREE DAYS.