5 = Victim

If 15 is the murderer, then 3 has to be a victim (from 35’s clue). If either 20 or one of the numbers in the rightmost column is the murderer, then the sum of the victims in the top row is even (from 5’s clue and 28’s clue). If the sum of the victims in the top row is even, then 3 will have to be a victim. No matter who the murderer is, 3 has to be a victim.

Done