This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Transformers: Shattered Glass
Megatrons' death. What makes it worse is that it didn't mirror any one of Optimus Prime's deaths. He didn't even have a choice in the matter, he was murdered by one of his own men!
Despite the Narm, Starscream's reaction was palpable.
Also Nightmare Fuel, at the end of "Invasion", Ultra Magnus manages to destroy an entire universe. Not only are the Heroic Autobot's and Deception's appalled by the scope of Magnus' plan. Even the normal-verse Decepticon's are disgusted and horrified by what the evil Autobot and his men had just done.