Malcolm doesn't flinch when Sherlock advances on him. He's taken on men who are taller than him many times.

"I'm not some lover of hers," Malcolm hisses back. "I'm her fiancee. I would never knowingly put her in danger. Stop projecting your own behavior on me and let's try to find her."

Joan told him about the kidnapping, but not a lot of the details. They haven't talked about it and he hasn't pushed her to. As a kidnapping victim himself, he understands how she feels and why she might not want to tell him everything that happened.