I see. So Lettie's a Card Carrying Villian and a Slave to PR. Which is why she commits evil, revels in it, and wants you to know it. The latter is also why when she's wrong/misses something she shuts up and goes along with a misunderstanding to avoid her apparent evil façade from cracking.
Many of the nobles there [in the capital] believe that coming into contact with monsters corrupts your soul.
Oh, I bet that's what happened to Lettie as a child. She originally was going to grow up a Wicked Lady in name and deed, but an encounter with monsters corrupted her soul inverting her sense of good and evil resulting in her growing up to be a Wicked Lady in name not deed. It'd also explain why she's quick to collect more bodyguards and security, can't have monsters reopening the trauma.
Who are you calling little girl?! I'm sixteen, and I'm the ruler of this domain!
To a middle-age man you're essentially a third his age. It doesn't help that you look like a 10-year-old and haven't grown out of your chuuni-emo phase.
Weiss (after Lettie compliments him): "Oh, it seems my usual strength went up approximately four times my usual. What's going on?"
Must be love.
...Unless you get the same strength boost from the royal guard complimenting you. In that case it just means you're easy.