DrWho Points Out:
The reason these liberals think bad guys with guns are so invincible is pretty simple. They haven't accepted that all people are not like themselves. Consequently, when they think of what an armed citizen would do, they first think of what they would do if they had a gun in that situation. In other words, because they would crap their pants, they assume everyone else would as well.
Since they cannot imagine themselves standing up to aggression, they cannot imagine that someone else would either. If they could imagine it, they would have to stop being liberals, as defending one's self or others is inherently contradictory to liberal ideology.
If you accept that you can defend yourself against criminal aggression... then that changes your world view. It alters everything else permanently.
The Democrats are the Party of the Squishy... the permanently offended... the perpetually insecure and terrified. Their voters are those seeking to hide under their beds from the big bad world... They shiver under the skirt of Aunt Samantha.