In Season 1, Episode 17 of The Flash, we learn via flashback (oof, sorry about that) that Eobard Thawne, the Reverse Flash, killed Harrison Wells and his wife while stuck in the past and assumed Wells's identity. In the key scene, Thawne explains:
"In the year 2020 you and your wife, Tess Morgan, successfully launched a particle accelerator that changed the course of history. I need it to happen a bit sooner if I’m going to get back — much sooner."
The implication seems to be that if Thawne hadn't taken Wells's place, the particle accelerator would have still given Barry Allen his powers, but not until 2020.
So Thawne's choice seems to be:
- Lay low for 21 years and wait for the particle accelerator to be built in due course, or
- Kill Wells and his wife, assume Wells's identity, and devote his life to constructing someone else's life's work -- and still have to wait 15 years.
I can certainly understand wanting to shorten 21 years down to 15 years. But Thawne seems to be saying that he doesn't have a choice. Is there a reason why he couldn't have just waited it out for the extra 6 years? (In passing, it seems like there were other options as well — for example, assume a third person's identity, join Wells's and Morgan's team, and help move the project along even faster. Thawne, Wells and Morgan working together could conceivably have gotten it done in 12 or 10 years, right?)