Other people had similar issues, here was my reply to them:
On a new attempt, after your target dies, open up your notes (press TAB), to see the cause of death and time of death. Then find out who kills him and try trace the steps the killer takes and try to figure out how to stop him. (There are multiple killers, each with a few different ways of stopping them).
(One death might not leave a note, it seems like we forgot that. You should follow your target to discover what happens to him)