Actual answer? I bet they came up with the climax first, then probably some of the early stuff, and, as is often the case, things get a little sketchy as they try and connect the dots. Also logical inconsistency makes people more likely to fall for twist endings.
Buuut since I kinda enjoy the mental exercise of rationalizing/explaining stuff I'll take a shot. At that point in time he wasn't actually married to Anna yet so his succession was far from certain, and may have been looking to curry favor with her sister with the whole "don't be the monster they think you are" business. He probably figured he could kill her at his leisure later the wedding.
His hand was later forced because he knew he couldn't deliver a "true love" kiss. Revealing his evil scheme after that and leaving her unguarded to "die" are just tropes you have to roll with
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