While most may wish to translate sinat chinum as "baseless hatred", I prefer to translate it as "purposeless hatred". It is learning how we are to deal with what we may refer to as our more negative emotions that must be our goal during the Three Weeks.
To understand why I translate sinat chinum in this manner and to further understand the lessons that are to learned through this term, I invite you to read the following articles from the Nishma website:
Defining Sinat Chinum (Part One)
Defining Sinat Chinum (Part Two)
Rabbi Ben Hecht