So first of all I admit I can't constrain myself sometimes and I curse back when I'm being cursed.
But that's the thing: "curse back". Is it less blamable? No, it's still called cursing. But it's caused by other people swearing. And I know a lot of people are like me, they don't flame if something goes wrong, but when someone start writing all sorts of offensive stuff in the chat about them, they talk back like: "shut the **** up moron" or things like this.
And I have something more concrete as an example. I was in a game, playing as jungler. I saw the enemy Yasuo overextend and I though, hey, easy gank. But I walked in lane like a noob with less than half health and he focused me, ignited me and killed me, and then killed Lulu because of the red buff he took from me. I was sure Lulu would say something so I was ready to reply something like "Shut the **** up, you have smite as a Lulu mid" (yes, she had Smite). But she said nothing. So I said nothing. And after 2 minutes I ganked again and we managed to kill him, no casualties. And now that I think about it, if she had flamed, I'd have flamed, and then we would've tilted and we would've started to flame each other and things would've gone first. I feel bad to have even thought cursing her now. Of course, we then lost the game, because Ashe decided to go troll (typical LoL game, huh?), but we didn't flame her either and it wasn't a bad game overall.
But the thing is, I think a lot of people who flame do it because they get flamed. And a flame war then starts. So don't start it. Don't be that guy. I try my best to never flame my teammates even when they fail. If they cooperate and are nice people, then even if we lose the game is still fun and you learn stuff. Because I know that when I fail I dislike being flamed. It does happen to flame sometimes tho, when someone really pisses you off. I think everyone's flamed at least a few times. But if you focus on your game, it's way better this way. If no one starts the flame war, no one will continue it. That's logical.
EDIT: Also, I know this isn't for "Help & Support" but for some reason that's the only thing that appears in the list. Rito pls.