Why does Wonder Woman kill terrorists in her first action scene in the Snyder Cut? Isn't she only supposed to kill if necessary?

In general, Wonder Woman does not have a rule about using lethal force.

I would assume that since dozens of innocent people were threatened and the terrorists had planned to blow up the entire building, Diana allowed herself to use lethal force.

Bear in mind, the Snyder Cut was originally filmed in 2017, before the script of Wonder Woman 1984 was even written.

I don't know but apparently the movie is not doing very much for the stock market because the movie stock is falling like a rock

I don't know, maybe because they were trying to kill a whole lot of people.