The sudden interference from the older man isn't something that Terezi was expecting. If this was one of the timelines presented to her, she certainly hadn't paid it much attention. The turtle's dodge would have avoided her initial attack, but the man shoves Donatello further out of her range, just as was prepared to go after him again.

Irritation flares up in her chest. She doesn't appreciate this wrench in her works. In fact, it scares her a little that she hadn't Seen this coming. There's something strange about it, and in this sort of situation, strange is dangerous. There's only one thing to do.

The shove has left the man wide open, so Terezi snaps her weapon back to her and pivots on her heel. She hears Donatello shout, but it doesn't stop her from whipping her weapon around and aiming a sharp crack at human's ribs. If that doesn't put him down, she's more than willing to keep going. It's better than letting him throw her plans into chaos. It's better than having Karkat or Cuthbert die.