Spoiler Below Since two of Theo's statements (10 & 12) are co-dependent, they must both be true & 11 must be false. Thus, Theo & Margaret are both innocent.
Since Theo is innocent, 9 is false, so David's other two statements (7 & 8) must be true.
Since 8 is true, 15 must be false, so Margaret's other two statements (13 & 14) must be true. Thus Judy is guilty.
And remember, "I'm-a Luigi, number one!"