You're right that he acted like a jerk, but most real people have their worst moments when they are at the very end of their hope. I actually love that about the story. I think it drives home just how desperate George was. It is something that makes him relatable to a lot of people. A lot of people have felt those desperate moments and behaved poorly. It's a Wonderful Life portrays a very good man with the nuanced understanding that even good men have very bad moments when they are under pressure, but those bad moments don't erase their impact or goodness.