More than two years ago, a jury acquitted Wardel Strong on charges of attempted murder. On Sunday evening, on a pathway in Harlem, he was shot and killed. His family believes that he was killed by people with ties to those he was accused of shooting. The police, who have not named a suspect, are inclined to agree. ''They think it was a little street justice,'' a law enforcement official said. Regardless of whether Mr. Strong, 28, helped cause his own death, family and friends said he was the type of person who would not avoid a fight. Mr. Strong's mother, Vanessa Strong, 48, said yesterday: ''He wasn't a bad kid. He wasn't the type of kid to look for trouble, but when trouble came to him, he didn't back down.'' His grandfather, George Strong, 76, agreed: ''Wardel didn't back down from anything.'' People in the neighborhood even said he was called Mad Dog. In the housing project at 125th Street and Amsterdam Avenue where his grandfather lives and where Mr. Strong lived for a while after