Transformers Movie) was when he took advantage of Megatron's damaged state and threw him out into cold space. It seemed like Starscream would finally become the new leader of Decepticons.
Source of all images above: Starscream (G1)
Unfortunately for him, Megatron (resurrected as Galvatron) came back and killed him.
Source of image from: Possession - Transformers Wiki
Amazingly, he came back from the dead possessing the body of that twitchy insect, Waspinator from Beast Wars. In an episode called Possession, he makes up a story about his death and that he died a hero only to come back and serve the new Megatron.
Everything was going according to plan, until he was betrayed for the first time by Blackarachnia who saw through his little deception. Instant karma.
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