Why did Thawne need the particle accelerator to be built 6 years earlier?

Question

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?)

Answer 1

Shortening his wait by several years seems to be his main motivation

The only real dialogue we get about Eobard Thawne's motivations are what you quoted. However, I think that the last part is key.

I need it to happen a bit sooner if I’m going to get back — much sooner."

The issue isn't getting back to his own time, the issue is getting back "much sooner". In other words, not wanting to wait as many years.

By pretending to be Harrison Wells, he can encourage Barry to develop his powers more quickly

At several points during the season, Eobard Thawne (under the guise of Harrison Wells) encourages Barry Allen to run faster, and teaches him techniques like phasing through matter, which he is especially capable of due to his firsthand experience. It's quite likely that the original Harrison Wells would have been less concerned about Barry's developing powers. Thus Eobard Thawne might have needed to wait even longer if he just waited for Barry's powers to develop naturally.

Pretending to be a third person is more risky and provides less control

Taking the identity of a third person to work on Wells' team means that he has a greater chance of being discovered, since Wells is smart enough to potentially realize what's going on. Say that Eobard Thawne proposes a particle accelerator design, and the real Wells realizes that it will explode. Well, then Thawne's out of luck.

There's also the problem of not having the final say because Wells outranks you.

Eobard Thawne: You can do it Barry! Run through the wall!

(Real) Harrison Wells: Are you crazy?! Barry, come back to base. We'll capture Leonard Snart another day.

Thawne doesn't care as much about preserving the timeline, so long as he gets what he wants

As we discover in Season 2 of The Flash,

Thawne is willing to kill future Barry out of anger, and is more concerned with him causing a Time Wraith to show up

Similarly, Season 2 of Legends of Tomorrow shows that

Eobard Thawne is willing to rewrite all of reality itself in order to serve his needs.

Really, his mentality to time travel is "mess with time to serve your needs, so long as you don't get caught". Or at least, for something desperate as getting home or preserving his own existence.

Answer 2

We can't assume that Thawne knew that it would be 16 years. He may have expected that with his future knowledge that he could accelerate the development of the Accelerator so that it would be only a five year wait. In other words, he wanted it done as early as possible so he could go home as early as possible, but it turned out to take 16 years.

Also by supplanting Wells, Thawne gets to be Barry's mentor, which fits into Thawne's disturbed obsession with Barry.