"You say that you're a loyalist, but you're really a sadist that found a convenient cause allowing you to fulfill your own dark wishes. And if you refuse to change that, then I won't allow you to poison it any longer."

Megatron pulls his arm out of that grip, and goes in again, this time clenching the badge and tearing it clean off Tarn's armor.

In actuality, it was part ideology and part disciplinary. The fact of the matter was that he had told Tarn to not harm the Autobots, and Tarn responded in turn by ripping up two and leaving them at his doorstep. Megatron had long since learned words were meaningless without action, and so Tarn would face the consequences of his choice.