Do humanists believe that humans are born good but due to some external factors are driven towards bad deeds?
You ask a really good question (and very complicated!). But to make sense of it, we have to think about what people mean when they use the phrase “inherently good” as applied to persons.
In one sense, we use the word “good” to just mean “valuable.” I take humanism to be a worldview that holds that human beings are inherently valuable. But this is probably not the sense you’re concerned with.
We also say that persons are good when we want to talk about their character. In this sense a good person is one that tends to do the right thing for the right reasons. In this sense, babies have neither good nor bad character – they lack the capacity to choose the right action for the right reason. Given normal human development, and a normal human brain (see Bo’s point, above), we can develop a good character.
Lots of great work has been done on how we develop moral concepts. One nice recent study is “The Ontogeny of Fairness in Seven Societies” in the journal Nature, Dec 2015 (truly, there is so much written on this). The authors conclude that our ability to be averse to inequity when we are the person receiving more than our peers develops later in age than being averse to inequity when we are the ones getting less. If this study is good, it’s a partial answer to your question: we are born with an innate capacity for fairness, and culture plays a role in how it’s shaped.
There is one last point I need to make. Your question is really good because wrapped up in it is one of the reasons I left religion. Consider the doctrine of original sin: people are born inherently bad in the sense that they have inherited responsibility for an act committed by the alleged first human beings. But, there is a problem with this view: such babies can’t have chosen that act for themselves because they can’t “choose” any actions. And in fact they didn’t even perform that act (and evolutionary theory tells us there is no single “first human”). Thus, as a humanist, I reject this view as deeply confused and even morally repugnant. I prefer to use good studies and careful reasoning to decide how we develop our innate moral concepts and try to create a world in which the most number of people possible can develop their moral capacities to their fullest.