It is this same overconfidence that killed Arsenal in the first leg. They underrated Porto. Underrating teams from other leagues is something premier league teams and fans have been known for. Arsenal really thought Porto would be a very easy match, a walk over and they were wrong.
Arsenal didn't play well, so I can't say they didn't deserve to lose. They had only one shot on target.

This is the same thing Arsenal fans think about the second leg. They feel because they're at home they would win the match. It is still not going to be an easy match for Arsenal. Arsenal still have a chance to qualify for the next round but they have to take their chances and not underrate Porto.