PSA: Who not to be, and what not to do, or what to expect.
Now, before you upvote or downvote this post, take a second (or hour) to really read this post with as little bias as possible.
I'm far from perfect. I wouldn't consider myself toxic, but when someone is extremely aggravating, i.e. they ask for multiple reports, spam pings or resort to ridiculous insults- "I hope your family and you gets cancer, lol what a beta, or any racist remark, etc" there are few times when I decide to go off on them with little to no restraints. I know I need to work on my ability to tolerate insults in-game, but let's all be honest, this wouldn't even occur or even be a problem for majority of us IRL.
Let's be really honest. Most of us are far from perfect. Many of us "non-toxic" players or people who haven't been officially restricted/banned yet, are either A. lucky or B. not as non-toxic as we think we are. Many of us enjoy rubbing it in those who have failed to uphold Riot's somewhat absurd morality standards, but the irony is that that is just as bad. It's kicking a dead horse in the mouth. If you think you're doing otherwise with comments such as, "well- you shouldn't have done it in the first place!" then you're... not fooling anyone but yourself and those who are just as simple-minded. It's stating the obvious while offering no criticism and only judgement. It doesn't even matter who or what the person did, most of us automatically trust and assume that they must have deserved it despite any circumstance. As much as it goes to serve your ego, it doesn't remedy the situation in any way. If we're going to talk about preventing toxicity, then we should follow through with our actions, and not just our words- as we all know actions speak louder than words.
Alright, let's talk about in-game rules/behaviors now. When someone picks something that you disagree with i.e. Alistar JG or Blitz Top, do not threaten to report or automatically assume that the person is a troll. Especially if you're doing poorly. Take a second and think about what you're saying before you say it. Let's say you're 0-2-9 as Yasuo and Blitz Top is 10-6-2 with a build you disagree with and a role you also disagree with. Do not complain, flame or ask for reports against the top laner because it makes you look like a fool. The pick itself has no relevance to your performance, and even if it did, the replay would've shown what actually happened.
Let's say someone made a really stupid mistake (flashed into enemy team for a greedy kill). Don't freak out. Don't ask "why did you do that". Chances are, they already knew they made a mistake as soon as they clicked the button. Chances are, they're already beating themselves over their own mistake. You/us worsen the situation by calling them out on it. You should also be prepared to receive the same criticism. i.e. If you're at 12 sated stacks at 25 minutes, don't go "........" whenever someone dies, it doesn't look good at all.
Long story short, refrain from using any negative remarks or comments that could lead to a negative situation. The golden rule of thumb is, if you don't want something being done to you, don't do it to someone else. Seriously take that into consideration. It would prevent a lot of circular toxicity.