Faith in humanity is a pretty tired phrase.
At any rate, you should try to keep in mind a few important psychological factors going on.
Namely, negativity bias
. It just means you're only going to focus on and remember the negative, not the positive. When people act polite, when they do what they're supposed to do, you don't pay attention to that. You're unlikely to remember the person who held the door open for you a week ago. But you will remember the people who broke the social code, the people who were rude and impolite. You might still be infuriated by them weeks after. Hundreds of people have done right by you in your life, but you're only going to focus on and remember the select few who did you wrong.
The media is also partially responsible. Violent crime is actually lower than it's ever been, but the media is going to focus and obsess over every aberration to that statistic. Especially because there's no way for the news to be divisive, because they're just reporting on the latest rapist/murderer/whatever instead of a policy initiative or something.
Humanity is objectively at the best point it has ever been and it is only getting better. The unfortunate thing is that the more rare the bad things become, the less used we are to them, so we become ever more shocked when they finally do happen. It's just a problem with human psychology and perception, not with the morality of the human race in general.