Criminal Justice major chiming in, you're wrong, you're only considered a murderer if you commit murder, which by definition is killing someone with the intent to kill them (not by accident, or self-defense, or any other reason you could possibly come up with). You are the accomplice to murder if you help plan the act itself, but not contribute to the murder in any other way than planning it. If you help in the actual murder however, you are also considered a murder (which is what I'm assuming you're trying to imply). So yes you would be considered a murderer if you helped attack them.
I'm not saying leona is a bad champion, nor should you build as a support all the time (there are instances where you have to build differently) but for the most part (except for maybe you, along with a few others) supports aren't gunna be doing much carrying in games in terms of leading the team to victory. (Also when a support tries to carry at the start of the game, it usually ends in the adc getting bitchy and afking or trolling...not that they wouldn't have done that in the first place, but besides the fact, if you go into a match as a support, unless it is necessary to act as something different, you should stick to your word).
It doesn't matter how good your focus is, if your teammates are a bunch of idiots you probably won't be carrying them to victory as a support. That's the sad truth. Unless you're using champions that provide massive pressure on the map, along with possible stacking that makes them great supports along with a great threat normally cough
. Correct me if I'm wrong, but for the most part, when building the way a support normally does;
choose one of the 3 items..
Then you have whatever boots you want to buy, along with maybe 2 items of your choice
it doesn't leave much room for building to carry. Also taking into account runes and masteries, it really depends on the champion I choose for support, along with the team I match up with, but 9 times out of 10 I've found building the way a support should build (meaning building for saving people's asses rather than building for straight damage) will help win more games.
I always like to plan for the worst, so even when doing shitty, I'm still helpful to the team, where if you build for damage, and it's not enough, you might end up being completely worthless. But that's just my point-of-view, if going straight aggro is your way to succeed, who am I to tell you to do anything different, I'm definitely no expert to this game, nor will I ever be.