Because I am capable of being horrible. Here's a take on why Tarrlok said 'It will be just like the good old days' and Noatak's tear.
….
He's looking at his lap, before his head turns to the side and wobbles a bit before his hand reaches out for a glove on the second shelf. A slow move, under watch, unnoticed. Now, if only he was the one moving his own hand.
But he doesn't fight against this. He had consented when he had walked out the cell door. For better or worse, he was with his brother again. The joy of seeing him alive was tempered by the sweet despair of seeing him so changed. Seeing his beloved brother as the person he viewed as being so far apart from their childhood days. They had reached a mutual decision; even as Noatak talked about starting over in a new world, Tarrlok's other hand voluntarily reached over and unscrewed the gas cap lid.
"It will be just like the good old days." He promised. He doesn't know what he's promising. Perhaps he promised that they are brothers again, that they would remain brothers, and that they care for each other. That they say goodbye together, and this time, they leave when his older brother asked to.
His hand felt like a poison (from the blood bending) taken to ease the passing (into their end) as the glove slipped on. His face is dry, but he knows somewhere in there, the caring boy Noatak was crying as a tear slipped by in the wind. He always was the older brother, looking out for him, even in the city.
Republic city had had oppressors, but it wasn't bending that was the problem. No, even without his bending Yakone had managed to shape them into tools. It was people that were the oppressors. Bullies that used their bending to intimidate people. But Amon had made no exceptions. Perhaps, if he was still in office, he could add chi-blockers to the metal bending task force.
But that time was past.
Together, the glove sparked.
…
Fin
A/N: Now, I know that in all probabilities Tarrlok made the decision on his own and they both died. But this idea just jumped out at me.