One I already stated. The violence and carelessness will make our world such an unsafe place that we dwindle ourselves down to nothing.
Another way is through greed. If there are monopolies and unfair treatment everywhere, there will be a lot of people who starve to death or die from diseases because they are too poor. There will also be a lot of people who are just barely surviving, and those people will probably revolt against the corporations. This will cause more death. Also, greed causes selfishness which is terrible for the general health of society.
A third way is through technology. The more we keep rely on it, the more problems there are. It is very hard to contain the internet and regulate it. No matter how advanced a anti-virus system may be, there are ALWAYS ways around it. Code for any program is finite, and so it cannot protect against everything. In the future if the entire economy relies on the internet, if it happens to collapse so will the economy. That cannot be good for mankind. Also, with technology and expantion of towns and cities, we are killing the environment. Remember the ozone layer and global warming? How about cutting down the rainforest and killing species and our oxygen supply? And then there's the problem of pollution. One thing that is good is that the more intelligent we become, the more we are aware of the bad we are causing our environment, and the more groups that fight to protect it.