The story of Walter West is probably the most complicated, as it involves such a great depth of background.
Linda was kidnapped by Abra-Kadabra. When Wally went broke the Speed Force, he used Linda as a link back to our world. If she exists outside of all dimensions, Kadabra reasoned, Wally won't be able to get back. And so she was brought to a place outside of all reality, and the memory of her was erased from everyone who might be looking for her. But she escaped, and fell through Hypertime, looking for her home world. She thought she found her home, and thought she found Wally. Except this Wally had blue eyes. Not green.
Walter "Wally" West was the same as Wally in every way, but lived in another dimension. Literally. He lived in another branch of Hypertime, a dimension where he was not able to save Linda from being killed by Kobra. It drove him crazy, and he became tough and merciless in dealing with criminals. Seeing her alive again drove him crazy and he was so intense and possessive that he frightened her...
Wally West, in the meantime, was fighting Cobalt Blue, Barry Allen's apparent twin who hated his brother for being the Flash and having all the glory, so Cobalt Blue killed Barry before Barry could save the universe in The Crisis. Wally ran back through time and saved Barry and then defeated Cobalt Blue by bringing him into the Speed Force, sacrificing his own life.
And back to Linda, who was saved from Walter's craziness by Wally, who wasn't dead after. He'd pinpointed her location in Hypertime and came to be with her.
Walter was amazed. Amazed by Wally's love for Linda, and his heroic attitude, Walter was turned back into a kind human being. And then, Abra Kadabra killed Wally and Linda before Walter's eyes. Walter accelerated his own metabolism to heal the wounds Kadabra inflicted, and aged 10 years. He vowed revenge on Kadabra and, with a new costume, traveled to our timeline to seek it out.