Why did John Steinbeck give Lennie a soft-side and a harsh animalistic side? What is the purpose of that?

I think that part of the reason Lennie is constructed in
such a stark and dualistic manner is to reflect how human beings are complex.  In the
most unique of ways, we, as human beings, are like Lennie.  There are moments of
tenderness within our own frames of references as well as moments of intense savagery. 
This encompasses our human experience and is representative of Lennie's predicament.  He
holds intense amounts of strength, something that cannot be restrained or controlled
when the circumstances are aligned in a particular manner.  Yet, his character is one of
overall goodness.  It seems to me that Steinbeck wants to bring out human complexity in
the idea of how good people can do bad things.  By any definition, Lennie could not be
seen as a "bad" person.  Yet, he does do bad things.  In the same way, George could not
be seen as a "bad" person, but he does a "bad" thing in shooting Lennie.  In the end,
Lennie's compassion and his intense power of destruction are both a part of his
character and in displaying him in such a manner, Steinbeck seeks to make a statement on
what it means to be human.