Thank you @joseph! I think that humans are supremely good at rationalizing things, especially their own behavior.
It's the reason we look down on someone for being rude and dismissive towards people in the service industry (waiters, baristas, etc) as bad people, yet when we do it's just because we're having a bad day, or we're in a hurry for something important.
It's the reason why we admire good samaritans, the sort of people who would pull over to help someone on the side of the road. We tell ourselves that we're the sort of people who'd do that too, if we weren't worried about the possible danger. The truth is that there's always risk, even for the original Good Samaritan, but we invent these reasons for why we don't act like a good person yet still consider ourselves to be one.
I don't think we're evil at heart or anything, just that, barring the rare psychopath, there don't exist cartoon villains who wake up every morning and say "Today, I'm going to do EVIL!". The vast majority of all evil acts in the world were done by people thought they were doing, if not good, at least justifiable things. Not good, but the Greater Good. The ends justify the means (which Machiavelli never said by the way, it's a loose translation, but I'm getting away from my point). However you want to say it, it's the same basic idea.
Almost all of us consider ourselves good at heart, and many of us even actively try to be good. But whether we are or not is a different story.
TL;DR, if you judge the goodness of a person by their intentions, then yes, most of us are good at heart. If you judge by our actions and their results, then no, most of us aren't.
P.S. Please don't think that I believe that this doesn't apply to me, or that I think I'm superior. I would say that I think I'm not good at heart, I'd even go so far as to say I know it, but I don't grok it. At the end of the day, there's still that same voice in the back of my mind making those same justifications, and I rationalize myself to sleep every night.