Finished last week's episode of Supergirl today, and this really appears to be the theme for this year.

Flash: DeVoe kills twelve people (at least), and will kill more. Instead of taking him out when they had the chance, they let him complete his goal.

Arrow: Rather than take out the Dragon, which has already shown to kill on at least two on screen occasions, they let him complete his goal.

Supergirl: Imra tells Supergirl that Pestilence evolves into Blight, killing millions. When given the chance to take her out, they let her complete her teamup, and most likely their goal.

Overall, I think it's a weakness in the series. Sometimes killing is necessary, especially to save innocents.