A superhero is only as intriguing as that one main villain that constantly tests their mettle. Batman has The Joker. The Avengers have Thanos. My diet has Cool Ranch Doritos. And The Flash has ... Reverse Flash.
OK, so the nomenclature is a little too on-the-nose, as is the bad guy's costume. It's literally just the inverted colors of Flash's get-up. So why does Reverse Flash hate the heroic speedster so much?
Why does Reverse Flash hate The Flash?
When it comes to the CW show, it's not really spelled out as much as it is in the comics, but the gist of it is probably best summarized by Redditor DrWhoBruh who writes, "He hates the Flash because he was so damn obsessed over him that he started to become like him, only to realize his destiny is meant to be his greatest enemy, so that really ruined his life and made him hate The Flash."
Because both Reverse Flash and The Flash are speedsters (more on that later), they can play fast and loose with time. It's understandable why Eobard Thawne (Reverse Flash) would feel some type of way about a hero that he idolized, only to discover that his fate basically dictated that he was supposed to be Barry Allen's mortal enemy.
Reverse Flash's origins.
Buckle up, because things are about to get granular and geeky. Dr. Eobard Thawne was a nerd from the future who idolized The Flash. So much so that he even replicated the accident that gave Barry Allen his powers. He was able to not only complete this task, but he used the Speed Force to travel back in time and visit The Flash Museum.
Here's the thing, though. Because Eobard was such a Flash nerd, he knew all about The Flash's nemesis: Reverse Flash. The thing is, because he lived in the future, no one really knew the villain's name. But back in time at The Flash Museum, Eobard discovered that he himself was, indeed, Reverse Flash.
This enraged the doctor: I mean how would you feel if you found out that the person you were destined to be not only a villain, but the rival of someone you worshipped?
Eobard's hatred for Barry has manifested in a variety of different storylines where he's done some crazy things just to spite the Speedster.
How is Reverse Flash still alive in the CW show?
The Flash show deals a lot with time travel, and there are subsequently a lot of rules associated with "The Speed Force." The reason why Eobard is still able to live is because he's a Time Remnant.
So, what is a Time Remnant? The Arrow Verse wiki describes it like this: "Time remnants ... are a phenomenon that occurs as the result of meta-human speedsters' time travel abilities. Due to possible time ruptures and changes brought by the speedsters traveling (or staying) further back in time ... the time remnants are copies of the original speedsters preserved via the Speed Force ... having to appear in the new timeline in order to prevent further time paradoxes."
So, all of that essentially means that a "copy" of Eobard Thawne has been preserved by the Speed Force in order to keep the fabric of time intact. Xandermcc on the Arrow Fandom site gets it down pretty well: "Thawne is a time remnant. If Eddie erases Thawne, he's never born to motivate Eddie to shoot himself in the first place. It's a temporal paradox. The Speed Force fixed this by keeping him alive up to the point of his death by Eddie so the timeline remains intact. Therefore when Eobard accidentally shows up in Season 2 before murdering Barry's mom, he hasn't yet met the team and everything."
If it all seems like a lot to follow, well, that's because it is. You can catch new episodes of The Flash on the CW on Tuesdays at 8 p.m. EST to probably understand it a bit better.