What trial was he to face? Sweden hadn't charged him with anything. They wanted to interview him as a witness, against himself essentially, so they could use what he said against him if (not when) they might have actually charged him in the future. He offered repeatedly to be interviewed, first in the UK and later in the Embassy.

This whole situation was ridiculous and kafkaesque. Charge him or do not charge him, and if you do charge him then, any only then, demand extradition.