I just finished Kimi no Suizou wo Tabetai the movie as well as the manga. I found out that the main heroine (Sakura Yamaguchi) was found dead with a knife on her chest.
I’m wondering if she got killed by someone and if so, who was that and why did he/she do so?
My guess: I strongly believe that her ex-boyfriend murdered her.
Yes, the main heroine was killed by a random slasher unknown to the main character.
The murderer's name was never mentioned in all media (light novel, live-action film, manga, anime movie), but it's the random slasher that has been mentioned a few times in the story (in the newspaper, on the TV news). It's unknown to the main character as he said,
山内桜良は、以前から世間を騒がせていた通り魔事件に巻き込まれた。どこの誰だかも知らない犯人は、すぐに捕まった。
Sakura Yamauchi got dragged into the random slasher (通り魔) case that has been disturbing the society since then. The culprit whom I don't know was quickly arrested.
Since this is a random slasher case, there's no particular reason why they were attacking someone: she was just happened to be in the wrong place at the wrong time.
That said, the point of that scene is not about who the murderer was or why she was murdered, but why it happened despite the main topic about "pancreas".