Thor: The Dark World

Jane Foster is infected by the Aether (the Reality Stone). Malekith the Dark Elf wakes to claim it. Frigga, Thor's mother, is killed. Thor and Loki reluctantly ally; Loki fakes his own death, then secretly deposes Odin and rules Asgard disguised as him.

The Aether is handed to the Collector for safekeeping, since keeping two Stones in one place is unwise. Thanos will take it from him.

Who is the Collector? Taneleer Tivan, one of the oldest beings in existence — an Elder of the Universe, and brother to the Grandmaster you'll meet in Ragnarok. He collects rare species and artefacts on Knowhere, a mining colony built inside a severed Celestial head. He appears here, in Guardians of the Galaxy (film), and in Infinity War (film).

He does not support Thanos. He accepted the Aether for safekeeping fully intending to add it to his collection, which is exactly what a collector would do.

Thanos takes it by force — and the scene the audience watches of Thanos torturing him is an illusion. Thanos already had the Stone, and had already killed him.