The relationship between master and student, or mentor and mentee, is often potrayed as a positve one. The mentor teaches the mentee everything that they need to know and protects them from danger, while the mentee feels a sense of love and gratitude. Sometimes, though, things go horribly, horribly wrong. One of the most dramatic instances of this are anime characters who slayed their master.
In some cases, this happens because the mentor turns out not to be such a benevolent person after all. That's the case with Domon Kasshu's mentor, Master Asia, both of G Gundam. Though he eventually does recant his behavior, Master Asia does try to destroy humanity, forcing his student to put a stop to him. In other cases, it's the student who betrays the master - Blackbeard was using his mentor Whitebeard to gain power, and was happy to dispose of him when the time was right. Sometimes, though, it's no one's fault. Kiritsugu Emiya of Fate didn't want to take down Natalia Kaminski, but it was either that or unleash demonic bees onto the general population.
Which of these events got to you the most?