I've encountered some rude people, some trolls who deliberately sabotage the game, some people who are fine when winning but become rage monsters when behind, and certainly people I'd never choose to play with again. But you know what? That's been the exception, not the rule.
In truth I've met some pretty cool people in this game. My "friends" list on my main account is a couple hundred people long - all people I enjoyed playing with and look to play with again. The majority of players that I've personally encountered over hundreds of games with ten people in each game have just wanted to win and have fun (disclaimer, I'm referring to unranked Normals games, my understanding is things get a little more tense when LP is on the line - I'll find out this season).
I'm not going to tell you there are no toxic players in the game. There definitely are. But there are also a whole bunch of normal people just looking to have fun, learn, and (hopefully) win. Don't let the fear of the "bad" ones keep you from meeting and playing with the good ones.
To the original poster - like some of the other respondents, I also started out with exclusively AI games. Once I moved to PVP I never went back - it's a totally different (and better) game.