What sin did Tracy commited?

why dont u listen to the movie instead of asking questions like that. it tells you right at the end the answer

The way John Doe chose to represent the last two sins, Envy and Wrath, was unlike the conventional method he chose for the first five sins. The first five deaths are those guilty of the specific sins.

The completion of his masterpiece goes a little different.

The perpetrator of the 6th sin, Envy, is John Doe himself, and he gets punished for it by getting shot in the chest by David.

The perpetrator of the final sin, Wrath, is David -- and he gets punished for it by being forced to live the rest of his live without his wife, thinking about John Doe's masterpiece.

Geddit?

"Yeah, well, that's just, like, your opinion, man..."

Like others pointed out, she was just a means to an end for John Doe to manipulate her husband.

She didn't commit a sin, her murder was all about Doe, he was envy and he killed her out of envy. His punishment was he was to be killed by her husband.