The death of Maes Hugh from Full Metal Alchemist (2003 and Brotherhood)
There is a lot of different reasons for this. The first one is that he is like the Phil Coulson of FMA. While he wasn’t an alchemist, he was completely awesome in his own way. He apparently was great at knife fighting, which was something we only learned during this scene. Maes also knew every single important character up to this point. Colonel Mustang knew him forever which inevitably meant that Hawk Eye knew Maes as well, he took in the Elric brothers while Ed was still recovering, and Winry was invited to his daughter’s birthday party after he met her in the hospital. This meant that his death affected everyone and motivated them to move forward in different ways. Sound familiar?
The murder of Maes Hughes weighed on the conscious of everybody, but mostly Mustang. Deeply devastated by his loss, Mustang spent a lot of time and resources into finding this person and when he eventually did find the killer, there was hell to pay.