Re: What was Zedd's obsession with Tommy?
 

Once Tommy became White Ranger he was out of Zedd's reach. Since the bad guys couldn't mesh with his powers.

Re: What was Zedd's obsession with Tommy?
 

Zedd was sadistic and wanted to hurt the Rangers in the worst way. Taking their comrad away from the team again would devastate the Rangers. Look how much trouble Jason had dealing with it.

Re: What was Zedd's obsession with Tommy?
 

Its not a surprise Zedd would want Tommy on his side, who wouldn't. Tommy would have never joined him willingly. Zedd was the ultimate evil and would have loved to break Tommy away from his friends. Especially Kimberly, since Zedd knew it would make her miserable.