When people say don't be that guy, I think they are talking about this guy. Brandon Jacobs received some interested tweets. Turns out they were death threats. Check out this insane fantasy owner who took it a little too far:
That's some intense fantasy football right there. Seriously though, who takes fantasy football that far. I don't think that I would ever send a death threat to someone for not performing.
This is just another example of taking it too far. The account was deleted shortly after the tweet was sent out. Just remember not to "be that guy".