I feel that I have been relatively blessed throughout my coaching career in having supportive parents, both mine and my players'. Of course, you are going to get some parents each year that are not on your side on all things, but rarely have I had some that are intentionally (or unintentionally) trying to implode your team.
There are bad parents out there when it comes to having teams, most are very good, but there are some that need to reassess what they are doing when it comes to support of a player, team, or program. There are behaviors that I have observed in 20 years of coaching sports and will help me to be a better parent.
If parents are consistently bad mouthing other players, there needs to be some reflection done on their part. Most times those players are just doing what the coaching staff has told them to do and jealousy is a huge issue here. Unfortunately, there is competition and sometimes that competion goes back a generation or two. You can bad mouth the coach (though it isn't helping or changing anything...probably), but other players should be off limits unless it is about behavior issues.
Those parents who do not or rarely attend their child's games. I realize that many people are working and working and working, but your children should be a priority. That is why I will not continue to coach if I am missing my own children's events. I remember when I was a kid, my parents attended most of my events and I didn't think it was a big deal at the time, but looking back on it, it was.
If you are allowing the violation of team rules or worse yet, hosting "parties" in which team rules are being violated, as a parent, you need to reflect on what kind of example you are setting. This defiance of not only rules, but universally agreed upon good parenting skills is a negative that needs to be reflected upon.
Finally, there are parents who will not pay for items or help fund raise out of spite of the coaching staff or school. If your child chooses to participate in athletics, you should pay your fair share. Also, you should help in fundraising despite your feelings about the coaching staff as your child will benefit from the items that are bought. I do understand that times can be tight, but some communication would be helpful. Most coaches are willing to work with parents to make sure your child can participate in sports.
What is interesting about the "bad" parents is that they are highly defensive and will flat out lie for their children and their own bad bahavior. They will also justify what they do because they are dealing with a horrible coach, a jerk of a coach, or....well, whatever.
I probably deal with one or more of these kinds of parents every few years. It isn't a yearly thing, but it has existed and will continue to do so. I am so thankful for those parents who help with our program by getting their children to practice and games on time. Those parents who help in observing our team rules. Those parents who pay for their items and help us fundraise in a time when the economy is bad and jobs are hard to find sometimes. Those parents who will give you an encouraging word or email when things don't seem to be going the way we all wished they were.
I am sure we all have some "bad" parent in us for selfishness or necessity, but I think it is something that we need to look at. It is like any of our behaviors, we must reflect and then be honest in that reflection. Are we doing things to help or to hurt the team? Are we doing things out of spite or anger? Or are we allowing our children to participate, have fun, compete, and allowing for a positive atmosphere?