Nonono okay so I have a theory. So we all know that Hans originally intended to woo Elsa, right? And he only switched to Anna because he saw that that wasn't going to work out.
So, this is scene is before he sees Elsa. Because, of course, at her coronation, he sees her nervousness and all of the emotions she's trying to keep hidden.
I think that's when he realizes she'll be too hard to get to.
So, back to this scene (and the following dorky-smile under the boat), this was when he was still planning to charm his way into Elsa's heart. But he was so respectful to Anna in the meeting scene for the same reason he was so "kind" to the villagers later on- to keep up the act. Obviously, when she said she was a princess, he bowed. And then the adorkable boat thing happened. So I think, yes, at this point, he did really like her, but not romantically. He was thinking, "Wow, she's dorky and cute. Too bad I'm gonna go for her sister." That's what that smile was. Just "Oh, she's pretty cute." And I think he would totally be interested, if he didn't have ulterior motives
And then, at the ball, after he'd realized that he wasn't gonna get anywhere with Elsa, he remembered meeting Anna. How awkward and obviously smitten she was. How easy it would be to win her over. So then he goes for it. He alters his plan, adapts it to the new circumstances.
He does this later on, too, remember? More than once:
"I figured I'd have to stage a little accident for Elsa. But then she doomed herself, and you were dumb enough to go after her."
He saw it as an advantage. Elsa was out of the way, so he didn't have to worry about that part of the plan. She was already done for, right?
And after he locks Anna in the room to TURN TO ICE:
"She was killed by Queen Elsa."
"At least we got to say our marriage vows...before she died in my arms."
"With a heavy heart, I charge Queen Elsa of Arendelle with treason and sentence her to death."
And them of course, this:
"Your sister is dead...because of you!"
A lie, which leads to a vulnerable Elsa, which lets him do this:
Sorry for the rant. I've had that theory in my head for for so long now, and I couldn't stand seeing another post about being confused because of that smile. So, there ya go. My headcanon. Doesn't mean it's true, but it seems more logical to me than thinking that he was out-of-character.