Luckily, Luke is not an instinctively strong telepath. He can tell something he just said upset Vader, there's a flare of something subtle, revulsion, maybe, horror before everything vanishes in the familiar storm of rage and hate.

"You took them. Well, the Empire. Your elite troops, as I said, including Kreel. I have no idea what happened to his collection after I escaped, but there were at least... a dozen, I guess? That I saw activate."