Jessica doesn't drink blood. That makes her weaker too. She feeds sometimes but mostly bill makes her mainstream.
When Eric first met bill, bill tried to stake him. Eric said bill is strong for such a young vampire. One of the reasons he let him live. So age is NOT the only factor in strength and fighting ability
And they threw stabler in and killed him fast as a twist. You saw his name, they advertised him, you thought u knew what was going on. Then they surprised you. Lots of movies and shows pull this move. It's the opposite of when 4 main characters go looking for a monster. And they bring along no name guy #1. U know he's dead