This is based on opinion. Please don't list it on a work's trope example list.
Tearjerker / From Dusk Till Dawn
For the movies
Seth's asking Kate's forgiveness for the people he killed in Kansas.
”I left some bodies on the ground back in Kansas. Now, no doubt they were good men and true, but they were in the wrong place at the wrong time, and they stood between me and my freedom. So I did what I had to do. I did what anybody would do. Can you find it in your heart to forgive me?”
Jacob telling Kate and Scott that he has been bitten and will turn into a vampire eventually... but also that until he does, he will help them.
"I give you the peace in death, that I could not grant you in life!"
Remember the guy who's saying this, is telling it to his little brother... ouch.
For the series
This exchange between Richie and Seth.
Richie: I'm not her slave.
Seth: Then what are you, exactly?
Richie: I'm your brother.
The look on Seth's face when he watches Richie kill the Culebras in the labyrinth.