The only way to determine this is to consider the semantic relationship between these candidates and the verb "kill" (parse trees simplified, and only portions shown here): Although the PREDs of both S1 and S2 match that of the question "killed", only S1 matches "Lee Harvey Oswald" as the head of the logical OBJect.
for S1, the matcher awards additional credit to node [2] (Jack Ruby) for being the logical SUBJect of the killing (using anaphora resolution).