It is not what is seems

Why is it when we tend to know some negative things about a particular person we tend to let other ideas manipulate our own. Is it because we have weak perceptions about things or other tend to be so convincing that we easily doubt our own opinion?
There are so many incidents that happened in my life that I betrayed my personal views.I don't know what is really the reason behind those uncertainties. All I know is that I easily be manipulated by some factors such as convincing power of other people and the way they reason out.Well I think I expect too much that I will fail to reach my own standards and expectations.I also think that the way people behave will also matter. Like for example, I never expected that this person is bad because that person seems kind but then I found out that "this person" is evil. This creature is a reptile. He cares nothing but only his own survival. He is so selfish. He only thinks about his own desires without thinking he might hurt someone. And because of that I was able to switch place and reject what I used to believe. I totally changed the way I view him. What he did was so unjust and unforgivable. He deserves to suffer. He must pay.
"Things are not what they seem" and "looks can be deceiving." What we think are not what they are most of the time and we tend to stick with what we know and what we see not knowing that it is not enough,that is why we think we are right always. But the truth is we are not.