I am starting off this challenge by breaking my own rule: Adultery, technically, is not a way to die. But it was definitely a motive.
He was a young gold-digger (what, it's not like the 21st century invented the idea) called Kereki Gergely. Not much good is written about him in the chronicles: His alleged crimes included forgery, church robbery, and even murder. In the end, however, he was brought down by a much smaller crime: According to legend, he made the mistake of cheating on his wife.
Benigna was a wealthy noble lady from an illustrious family, at this point probably in her 50s, and famous for her none-too-friendly character. When she found out she had been cheated on, the deed could not go unpunished: She had her young husband thrown from the tower of their home.
(I did not type "dumped" on purpose...)
Other sources claim she did it because Kereki was trying to get his hands on all of her property. Whatever the case, she got away with it, probably due to her own and her previous husbands' good family name. The king did take away her property, though, and she never married again. I wonder why...
* Throughout the challenge I will be using Hungarian names in their original order - family name first.