Lennie wasn't too good to live in this world. Evidence:
He routinely killed in anger and for revenge.
At first, he says he kills the mice by "petting them" too hard, but later, Lennie admits to George that he killed the mice by pinching their heads after they bit his fingers.
He killed the puppy because it "made like it was going to bite [him]," so he smacked it on the head, killing it.
He killed Curley's wife because she would not stop screaming, and he didn't want to lose his job of tending to the rabbits.
If George had not intervened when Lennie grabbed the dress of the girl in Weed (by smacking Lennie over the head with a fence picket), Lennie would most likely have killed her as well.
The only justifiable violence in which Lennie engages is when he crushes Curley's hand.
I think Lennie is the perfect example of how the world destroys the innocent among us. Look at what Lennie had to do to adapt. He had to keep his mouth shut for fear his employers wouldn't want him, despite the fact that physically he was ideal for the job.
Our society moves so quickly and people in it are so jaded that they are skeptical of anyone who does things for purely innocent or altruistic reasons. Even today, someone like Lennie would be hard pressed to function on his own. He would still need someone like George to be his buffer to the people around him.
Everyone is skeptical of people who are innocent. We are so used to expecting that everyone is out for him/herself and we can't believe that someone can be as innocent as he/she seems. We want results, we want to be successful, we want to be rich. Those things aren't a result of being innocent. To get ahead in society, as most people see it, you have to be willing to do anything, walk over anyone.
Those people walked over and taken advantage of are often the innocent. They are tricked, cheated, hurt, forgotten in the modern quest for success.