Hime Shinjatta!

A serial killer sets out to assassinate a luxury-loving princess. However, upon arrival, he finds her flirting with a prince!? The prince tries to protect the princess, but in the process, accidentally pushes her, causing her to hit her head and die instantly. The killer, who didn't even have to dirty his hands, considers it an easy jobuntil the prince makes a shocking request!? (Source: Shonen Jump Plus, translated)