Basically because they had become their father. They had used their extraordinary bloodbending abilities to rule over people, had fought and lost against the Avatar and were fleeting to the Water Tribes to seek refuge from justice. What goes around it comes around, so he Tarrlok just decided to put an end to their legacy right there and then.
Connor Jones
Because shock value and the writers had no idea what to do with the characters in future seasons
Lincoln Diaz
you sound like a fucking retard
Liam Jackson
They're the sons of an abusive crime boss who fled and raised them to be his successor. Both tried to break away from their father's legacy in their own way: One of them became a politician and the other started a revolution. They both ended up doing illegal shit, causing a bunch of violence and becoming fugitives of the law. The last scene is them on the run by boat, just like their father. One brother reminisces about the good times and tries to feign happiness when he says the have a fresh start. Realizing that they had both followed the footsteps of the father they hated and will never change, the one brother puts on taser glove, opens the gas cap and blows up their boat.
Jaxson Turner
There weren't going to be any future seasons whe they were writing S1
Lincoln Wood
bump
Ryder Garcia
I hate LoK, but I will admit it had some touching moments that you wouldn't expect to see in a cartoon. Not enough to save it even a little bit. Still a festering pile of shit with only 2 or 3 good scenes.
Reuniting with his brother snapped him back to reality and made him realize that he'd basically become his father. He wanted to be a politician, not a terrorist or a criminal. Over time the power just went to his head. Instead of going back to society and becoming even worse than his father, with his crazy cult-leader brother at his side, he decided to end it.