There are many complexities in the Willie Horton story.
As a whole, this was an example of vicious mudslinging by the Republicans (it was reported that a leaflet in Illinois said "All the murderers and rapists and drug pushers and child molesters in Massachusetts vote for Michael Dukakis.")
1. Dukakis did defend the furlough program to the extent of vetoing legislation to exclude first degree murderers from it in 1976.
2. The argument that Horton attracted so much attention simply because he was black, is untenable. His crime was so horrific that he could probably have been put up as the centerpiece of an equally negative campaign regardless of his race (which didn't hurt of course.)
As an aside, it is not true that Willie Horton was first brought up by Al Gore. He actually asked a single question about Horton in a debate for the primaries, without mentioning Horton by name or his race.