He worked with rightists in what is commonly called the "opposition bloc" to destabilize the USSR, toward what end other than hating Stalin I am not even sure. Another member of this bloc was Yagoda, head of the NKVD and a butcher in the Great Purge, second only to his successor Yezhov (whose status in this regard I am unsure of). The bloc will make you lose years from your life trying to get consistent stories out of it if you research it.

As was mentioned elsewhere in the thread, he claimed that the Politburea were fascists and that the Soviets and Nazis represented a unified "camp" competing against the British-Aligned camp (though even in this distortion he recognizes that the Soviets hate the Nazis).

Trotsky flip-flopped on "Lenin's Last Testament", I believe initially recognizing it as the fabrication it was but later holding it up as real for opportunistic reasons.

Ironically, from what I can tell, he also flip-flopped on the "socialism in one country" thing (along with promoting a misrepresentation of it that exists to this day) after first supporting it, before even Stalin did, because once it became Soviet orthodoxy and Trotsky was ousted, it was just another thing to twist to paint them as devils.