If you are not good at cracking riddles, you might find it difficult to unravel the actual killer in this case. However, this riddle is not too difficult to crack as there are a lot of clues in the question pointing at the killer. Before going into the explanation of who is the actual killer in this question, know that the right answer to this is question is Jason.
Each number written on the calendar next to the dead person represents every letter that makes up the name of the killer. When you go back to the question, 6, 4, 9, 10,11 are numbers written on the calendar. The fact that they were all written on the calendar would tell you that each number represents the first letter of each month in a year.
For instance, the first number is 6, which means the sixth month of the year (June). If you do the same thing for other numbers, you will get April, September, October, and November. By arranging the first letters of the months starting from June, what you would get is JASON.