I don't think people wake up in the morning deciding to be evil. Nobody wakes up in the morning thinking "Let's see how much havoc I can wreak today". (There could be one or two exceptions). But I think they do make decisions on values and identity.
They may make decisions on other people. They may decide that "People are lazy". "People are stupid", "People need guidance", "People are ignorant", "My wife doesn't understand me", "My children are a burden", "I want to show my teacher I am significant"
They may also make decisions about themselves and what they want to do about it. "I want to get pleasure in life." "I am a great seduction artist", "I am a great leader", "I am a loser", "I am a winner", "I know a lot of things".
It's from these decisions that all other decisions and attitudes follow, even if reality doesn't bear out that truth. A person who thinks he is a great boss but doesn't have the people skills that match it will encounter difficulty when things don't seem to be working out. He may then rely on outdated skills or things learned from his parents or his ex-bosses. Or a person who believes he knows a lot of things that starts boasting, yet in reality people think him over-inflated and a bore.
I don't think HItler decided to be evil one fine day. But he did decide that Jews are bad and the cause of Germany's problems. He did decide that some people's lives are worth less than the cost of maintaining them if they weren't productive.
It's these decisions that shape you and I. Thinking about it. It's all the other tiny decisions that we make, the words we say, "You're bad", "You horrid thing", "You're an asshole", "I'm never going to make it" that make us who we are.
That's why again, it comes back to what we think. Thinking good thoughts is good but deciding and believing those thoughts are true is a different matter. I can think but not believe, "I can do all things through Christ who gives me strength". To believe in it is different. To think and know, "God loves me" is one thing. To believe "God loves me" is something else.