So a question....
When is it ok to be screaming, yelling totally biased fan? - I don't think we ever need to be totally biased. We are all naturally biased, but why can't we appreciate that the other team/players are playing well? Sure, I want my dd's team to win and play well, but I don't need to act like a lunatic from the sideline for that to happen.
While I agree with most of your post (except 1 - it does matter and should not be as bad as it is in most games), when we see the sideline at college football or basketball games, its not exactly supportive or sporting behavior toward the opponents or even our own players, when they make mistakes. The behavior is amplified in pro games. - very true, but at these college or pro games rarely is the "crazy" fan/parent two feet from the touch line. Ideally all parents should be far enough away from the field that the players and referees can only hear "noise" versus actual words/statements.
These players are all "children" of some parents and at what point do we hold players more accountable for their own performance and conduct on the field? - players should ALWAYS be accountable for their actions. I am not absolving them from this responsibility. In my time at the fields, I have seen only a few examples where the player's conduct was deplorable. However, you can witness deplorable behavior by parents at nearly every game.
Since you've mentioned your kid is U15, it is considered "olders" now and do we flip the switch? - I guess a flip does switch at some point. When my dd was U8, I sat on the sidelines whining over every missed call or missed foul. Now that she is older (and I am more mature about these issues), it does not bother me nearly as much.
Assuming that we're all talking about competitive sports and not rec, part of the player development is learning to win, lose, handle criticism and complements, unfair/bad calls and most of all deal with crazy (parents) fans on the side line. I've seen players talk back to other team's parents using 4 letter words and racial slurs.... - and this is wrong too. Players should focus on the game, their teammates, their opponents, their coach and the referees. Completely ignore the parents. I used to make comments to my dd during games and then she learned to tune me out and ignore me. Who would have figured an 8, 9 or 10 year old would learn faster than her old man?
Obviously I'm being rhetorical but don't make it sound like its all parents, coaches and ref's that turn a game into a sour situation. Players have lot to do with it... - there are wonderful parents, coaches and referees out there no doubt. It only takes one bad apple to spoil the bunch.