Turret's take increased damage when there's no friendly champion or minions on turret and enemy minions are pushed to it. It's not impossible for rengar to take a turret down by a quarter health in 3 shots in the right situation. Q has an initial base damage increase, and his buffed Q has a 100% ad steroid in addition to the base increase. Combined with some early ad runes, and the inevitable level 2 that occurs by reaching turret, and you can very well hit for around 300 damage in 2 hits. Not exactly a quarter, but the third hit + the minion hits that are happening at the same time can easily break the quarter health barrier.
Similarly, as the game progresses, if an outer turret hasn't been pushed down yet, rengar can easily take it down in less that 5 seconds from full health in the aforementioned scenario. Less if he had a good laning phase.
And ofc the community is toxic Lume. In any game where there's direct competition, people are toxic as hell. You just have to bear with it and just do your best. No amount of complaining will change it, and the more you dwell on it the more it effects you. Part of it is not just how you deal with them during the game, but how you deal with them after the game.
I had a habit in s2 to play until I lost in ranked. The instant I lost a match, I stopped playing ranked for a couple days. Reason was the game would always bother me, and I would perform worse afterwards because of the loss. Which would lead to another loss, and it would spiral downwards. Similarly, I stopped playing ranked the instant a game ended where I had a particularly toxic teammate(s). Even if we crushed the enemy, I would stop no matter what because just getting irritated by remembering them would throw me off just a little bit, and I wouldn't risk a loss over something so avoidable.
Admittedly, I only played maybe 15 games in s2 for ranked before I was content to just let my elo decay to 1400 because of guaranteed benefits (and before I broke my habit when riot announced that highest elo would matter, not current elo, when s2 ended), but I had like 12 wins and 3 losses before that happened. Almost all of them were playing as support, and I had almost 100% win rates for every other position as well. I would of had gold at least, probably higher, if I continued. What matters though is that you ultimately have the final say on how much a toxic player will influence you. Will you let him just ruin one match, or will you let him ruin your next 10?