Professor Zoom- The Flash Vol. 1 #324

Professor Zoom is a villain in the DC Universe that frequently interferes with The Flash. As the "Reverse Flash" his powers are tied to that of The Flash- as though they are the positive/negative sides of some energy spectrum.

Beginning in The Flash Vol. 1 #321 and ending in #325, the "Run for your Wife!" story arc sees Professor Zoom mounting a revenge attack on Barry Allen. After killing Barry's first wife, Iris West Allen, Professor Zoom was locked in an extra-dimensional prison for four years. When he finally manages to escape, Zoom plans to exact revenge by killing The Flash's new wife (Fiona Webb)... on their wedding day!

Professor Zoom is confronted by The Flash in an epic battle that spans entire states. To taunt his foe, Zoom carves an effigy of his dead wife into a mountain. Enraged, The Flash charges at Zoom at a speed that creates both a sonic boom and a rock slide on the mountain. Their fight causes a tidal wave in Florida and destroys a rocket in Cape Canaveral. 

While on the beach, Zoom writes his intention to kill Fiona Webb in the sand and takes off. Reading the message, The Flash takes off as well. Both speedsters are headed to The Flash's wedding ceremony- where Fiona Webb has pretty much given up that Barry Allen is going to show. When The Flash arrives he tackles Professor Zoom just in time to save Fiona. 

The speed and force of the tackle snaps Professor Zoom's neck and kills him instantly. The Flash killed someone in front of a crowd of people and, in The Flash Vol. 1 #325, he is arrested for manslaughter.

This is a significant story because, even though Zoom died, he still won in the end. Zoom broke Flash to the point of corruption and murder- very few other villains have ever been able to do that to their super hero opposites.