The Flash, Season 3

Barry:
No matter what I do to stop Grodd, he just keeps finding new ways to attack, new ways to destroy, and I... Iris doesn't think I should, but the only way this ends is if I kill him.

Harry:
"Kill him". That might be the most idiotic thing I've ever heard you say.

Barry:
Wait, what?

Harry:
Iris is right. You don't want to listen, go ahead, keep being an idiot.

Barry:
I don't understand.

Harry:
Don't. Kill. Grodd.

Barry:
Harry, you of all people should understand. You would've killed to save Jesse. You were gonna kill Zoom.

Harry:
Barry, there's a thousand things I've done... that I'm not proud of. Some of them recently. Every time I do one of those things, it makes it easier to do it the next time. I look at you, out there battling the darkness with honor and hope, and I'm reminded that there's always another way. Always. Killing Grodd is not your answer. Saving one life doesn't justify taking another. Find the other way.