Obsidian and his consort Strika have fought for Cybertron in countless battles... and won them all. Though he is sophisticated and canny, able to concoct strategies with layers upon layers, his purpose is single-minded. He defends Cybertron.
There is some fine print. Whoever happens to be in charge, whether a benevolent or malevolent force, in his reasoning, is Cybertron. Once power changes hands, so does he. He is loyal only to the planet, and perhaps his consort Strika, not to any moral ideals. As his fellow Vehicon general Thrust has remarked, "If you're loyal to everybody, can you really be loyal to anybody?"... a question that has given Obsidian pause.
While he was pretending to be dumber even than Tankor, his activation code (and, indeed, his entire vocabulary) was "obliterate". He dropped the word when the ruse was over.