apparently it's a meme related to Hibana, where a commenter named Akai (who the JP fans refer to as 赤井ニキ "Akai Niki") left a comment on Hibana saying "My icon has become terribly awkward", said icon being a guy aiming at the viewer with a sniper rifle. It's one of the top comments on Hibana, and Akai Niki has never changed their icon ever since making that comment.

EDIT: The character in the commenter's icon is Akai Shuichi from Detective Conan, and the commenter actually named their channel after Akai. The "niki" in the nickname "Akai Niki" is a Japanese Internet slang way of referring to someone as "brother"

(Akai Niki's channel)

Now there are comments related to Marshmallow going like "AKAI NIKI FINALLY DID IT" "DID AKAI NIKI SHOOT HER???" "AKAI NIKI SHOT HER" and other things along those lines. Especially in the replies and quote retweets of this tweet by Deco where he compared Hibana and Marshmallow's artworks.

idk i just found this very goofy so now im yapping about it

"Please act out your part like I want you to,

My "I love you"s were all lies too.

Swing and miss at NO, in that kind of love!

I really hated everything about you."