The thing is, chewing out teammates never helps. You don't need to make them feel bad. If they're legitimately trying and then they screw up, they probably already feel bad. If anything, you should be trying to make them feel better, so they maintain their confidence and don't throw the game.
When giving teammates advice, it helps to be non-accusatory about it, and couch the bad remarks with the good if you can. People tend to go on the defensive when they're called out on their mistakes. It's just human nature.
Consider the scenario where your jungler tries to gank a healthy enemy for you when you're out of lane -- and predictably drops the ball. You could give them some advice in one of two ways:
- Don't gank for me when I'm out of lane!
- Good attempt. Next time wait until I'm here, and we'll get them!
Both convey exactly the same information, and from the perspective of anyone looking in from the outside, both look perfectly fine. However, the former is more likely to sound like harsh criticism to someone that's just made a mistake, and thus is more likely to be ignored. The latter, on the other hand, keeps your teammate's confidence up and makes them feel like a part of the group. They're more likely to cooperate with you if you take that route.
A final cautionary word: take care to ensure you don't give your teammates advice that's overwhelmingly obvious to them. Do so, and they're likely to feel patronized and shut you out. By way of example, I was recently in a game where one of my teammates was towerdived and ganked. This was their third death in lane, so someone "helpfully" instructed them to hug the tower from them on. Good advice, to be sure, but with one problem: that player had already been hugging the tower for the past few minutes! Consequently, that player felt condescended to and proceeded to shut out any further advice that may've actually been useful.
tl;dr
Toxicity loses games. Be positive, respect and support your teammates, and everyone's likely to cooperate better overall.