What happened to the condom?

Tom says there was a condom, but what do you think happened to it?

Do you think Dan sneakily chose to take it off against Tom's wishes? In which case he's the true villain in all this.

Or did it break? In which case Dan is not really to blame for Tom's infection.

That doubt about the condom might explain why Tom realized it would be unfair to kill Dan at the end.