Tarrlok decided to detonate the speedboat, killing both himself and Amon, because he realized that they had no place anymore in the world; he wanted to have a last moment with his brother before dying together.

Why did Councilman Tarrlok kill himself?

In an echo of their childhood flight from their father, Amon found Tarrlok in jail and asked him to flee Republic City together. ... Ultimately, Tarrlok ignited the fuel tank and killed them both because he knew that their continued existence would only bring about more misery.

Did Councilman Tarrlok kill himself?

Soon, Tarrlok grabbed one of the Equalists' signature electric-shock gloves and held it over the boat's fuel source. He looked at his brother one last time and affirmed Amon's affections, by saying, “It'll be just like the good old days”. Then he set the ship ablaze, killing himself and Amon.