Angela Wexler's possible motive for killing Sam Westing might be to gain his inheritance. The money would allow her be independent, and pursue the career that she longs to have.
Intelligent and beautiful, Angela Wexler exists completely under her mother Grace's control. Grace has arranged for Angela to marry Dr. Denton Deere, and is revelling in the excitement of wedding preparations and plans for Angela's life. "Perfect Angela" (Ch.14), though, does not want to get married yet, but is too timid to stand up to her mother; whenever Grace wants something, "Angela does everything she says" (Ch.12) Angela chafes about being asked about Denton "all the time, as though she was nobody" (Ch.14). Although she is admired for her beauty, Angela is never asked about her own hopes and dreams. No one cares to know who she really is, and she is starved for attention.
Angela had gone to college for a year, and often wonders about the exciting lives her friends must be living now, with their careers. She herself wants to be a doctor, but does not have the money nor the will to make her dreams come true. It is possible that Angela thinks that her potential inheritance from Sam Westing might help her eliminate at least one of the impediments to achieving her aspirations.