In The Flash, why do they have to bring back Reverse Flash all the time to show he is a perfect villain?
And why is Flash also scared of Reverse Flash although both were speedsters?
Eobard Thawne, the Reverse Flash, is the archenemy of the Flash. The TV show can never get rid of him entirely for this reason - they need someone who can challenge Barry step for step, and Thawne is pretty much the only real candidate.
Barry fears Thawne for a number of reasons:
Thawne killed his mother.
Thawne has a greater level of experience using his speed than Barry does, and taught Barry much of what he knows (while under the guise of Harrison Wells).
Thawne is from the future and has historical information available that Barry hasn't lived through yet.
Thawne is a certified scientific genius, and much smarter than Barry is.
Thawne has no moral qualms about achieving his goals and will stop at nothing.
All of these factors make Thawne the most dangerous enemy Barry has, and one of the most compelling.
Both Barry Allen and the Reverse Flash’s lives are intertwined with time travel, friendship, and hatred. The point of keeping Reverse Flash is to show that Barry can never really be free of his connection to him. Reverse Flash is a representation of Barry’s past (his mother’s death), which means he lives in a timeline created by him (his mother didn't originally die), he even made Barry the Flash (by exploding the particle accelerator), and finally his daughter will be trained and mentored by him in the future.
Even if the show were to kill off the character for good, he will still be happily waiting in the future to meet barry and start the time loop all over again, yeah you heard me right time loop. Eobard learns about the Flash after the Flash is presumably long dead and then duplicates his powers and goes time traveling around only to see that his destiny is to become Reverse Flash (a villain), then goes back in time kills Barry’s mom and time starts over. All this being said, while Reverse Flash may not be more powerful than Barry he is smarter, and will most likely stay apart of the show until the end in order to keep it true to its roots of being a century-long struggle between two speedsters.