Just watched Akira (1988) for the first time, and I need answers!

I haven't read the manga, so keep the discussion to the film itself.

The movie's final scenes seemed to hint that the espers got their powers through human experimentation, while Kai seemed to hint that all humans already posses that power, but it was locked (amoeba analogy). How and why did Akira destroy Tokyo during WW3 and how did he end up being stored cryogenically? Why did the scientists even capture Tetsuo to begin with (before they found out about his powers)? Also, can somebody explain to me what exactly happened in the ending?

Overall, it was an incredible movie and the animation was fantastic.