Accidentally, through a mistake with her magic or just overestimating his ability to fight some villain.
Indirectly. Marco kills himself after getting rejected by her.>Brittney
She gives no fucks and pushes him into traffic after he pisses her off.>Janna
Also accidentally, through her fascination with weirdness and danger in general. Like, she uses Star's spellbook to summon otherworldly spiders or something and they end up eating him. Unlike Star, though, she might find how he died kinda amusing and joke about it.>Sabrina
Sabrina being herself, she gets comically injured. Marco being himself, and also hanging out with her for some reason, he gets hurt way more than she does and dies in the ER.>Hope
Behind those tiny little black eyes is the mind of a serial killer. Marco just happens to be her next victim and no one suspects a thing.>Chantelle
Nobody gives a fuck about Chantelle. She comes to school one day with a gun to ensure they all will, and Marco is one of the victims.>Mrs. Diaz
Her general inattention and sexual adventures with Rafael let Star and Marco do whatever they want, and leave Mrs. D. unaware that Marco Diaz, Only Child perished in another dimension and she could have prevented it.>Miss Heinous
Gets pissed off upon realizing he's not even a girl and kills him through attempting to make him one.>Queen Butterfly
Publically beheads Marco as a message to all of Mewni that racemixing must never occur.