Why is the significance of Steinbecks phrase 'George's voice was taking in the tone of confession' when George tells Slim how he once encouraged...

This significance of George's tone of voice in telling Slim about what he did to Lennie is that it shows that George has a conscience.  

At first, George thought it was funny to poke fun at Lennie.  Lennie would pretty much do whatever George told him to do.  So, when he told Lennie to jump into the river, Lennie complied.  The problem, however, was that Lennie could not swim.  Lennie nearly died.  

When...

This significance of George's tone of voice in telling Slim about what he did to Lennie is that it shows that George has a conscience.  

At first, George thought it was funny to poke fun at Lennie.  Lennie would pretty much do whatever George told him to do.  So, when he told Lennie to jump into the river, Lennie complied.  The problem, however, was that Lennie could not swim.  Lennie nearly died.  

When George pulled him out, Lennie thanked him profusely for saving his life, when it what George who put his life in jeopardy.  George realized at this point that Lennie was an innocent childlike person.  He felt horrible. After this, George stopped poking fun of Lennie and started to take care of him. 

George, in a sense, repented.  He felt bad, changed his ways, and realized that Lennie was a great friend. He was confessing this all to Slim.  This shows that George is a decent man, who will be faithful to his friend.