Radicalization tends to be a function of getting access to a group of like-minded people who agree with your angry/hostile/prejudiced views while simultaneously being socially cut off from moderating influences. The internet is able to connect isolated individuals with that group of like-minded people.
One of the dangers of Internet access to such is people who only know you online may feel more comfortable with encouraging your radicalization than if it was in person because they aren't in immediate danger, nor at high risk of being asked to participate, etc. It can seem inconsequential to agree with extremist statements. People who might hesitate to agree in person may not hesitate to agree on a discussion board.
There are good and bad things that come out of the relative freedom to speak our minds online. Opportunity for radicalization is one of the bad things that comes of it.