Did Luffy really kill Arlong? Hody said Luffy and the Straw Hats killed Arlong and his crew. As far as I know, Luffy never killed someone. I'm a bit confused
In anime, we are shown Luffy knocking out Arlong, after that it's assumed all of his crew is taken to Navy prisons but in the Impel Down arc, we don't see Arlong.

Luffy believes and like people who are really passionate about achieving their goal, even if they are his enemies. It is seen clearly, as Luffy always gives them a second chance to get better and achieve their goals. 

Even if we think Luffy killed them, why would Jinbei join the Straw Hat pirates? Arlong is like a brother to him though they have their differences. So from all this, we can say Arlong is not killed, at least not by Luffy.

One Piece is all about making people believing in themselves, their own ideas and pursuing them no matter what. These are a few situations which proved that:


Koby 
Zoro's fight with Mihawk
Dragon's Revolutionary Army
And many more .....