I understand the motivation behind faking the Duchess' death - so they wouldn't be hunted by the Prince - but was it really necessary to pin the fake murder on him?
Why didn't they just flee together after her 'death'? Why all the nonsense about 'calling her spirit up from the dead' or whatever in what appears to be a very mean-spirited revenge - that led to the Prince's suicide?
Don't forget that an innocent man was arrested for the 'crime' too! Hopefully he wasn't executed before the Price was 'exposed' for the 'crime' that never took place!
Crown Prince Leopold was a tyrant and brutal to women. Those two qualities alone make it OK to me for him to go down for the fake crime. If he wasn't a bad guy, he wouldn't have been convinced that he killed Sophie, the Duchess von Teschen. If whacking a woman on her neck with a sword was out of his nature, he wouldn't have acted so guilty.
I don't think the Prince's suicide was part of the plan though - I think the main idea was to get him arrested and to foil his plan to overthrow his father. Remember when Uhl says to Eduard that 'you' cannot bring down the monarchy, and he replies 'I can't'. In the end, ironically, it was Uhl himself who spoiled the Prince's plans! That was the best part of the movie. Getting the girl was just icing on the cake!
Oh God! This Parachute is a Knapsack!!! (Chandler Bing)
And let's not forget - if the Prince's plan to overthrow his father had gone forward - Sophie would have been in danger as a co-conspirator. Even if she was an unwilling one.