I can forgive the Leona, I wasn't there, but just looking at her score it's understandable that once the team was behind she would continue dying. She has to go in to do anything and lacks any escapes.
More in favour of her just being really bad though is also not finishing he gold item... unless you guys were facing Yasuo, Ashe, and Trynd that's not acceptable.
But yeah, that's what you are going to get sometimes. If you think you are better than them, you should be able to do consistently well and that tips the game in your favour. You need to learn that you can't just win every game, you are going to get bad players, trolls, AFKs (note, bad players and trolls are separate, don't call people who don't play to your expectations trolls). You need to learn not to expect the win, you should be in a mindset where you know you are able to win, but anything could happen. When you go in expecting to be fed wins, you just get mad when you lose.