First of all, viewing extremist material on the internet is still being influenced by others. There is no Big Byte Bang happening where this content explodes into digital existence from nothingness; someone developed and posted it. So for starters, we can take the 'self' out of 'self-radicalization.'
Second, 'radicalization' is something of a misnomer. One can hold radical beliefs without being a violent extremist. What does being 'radical' mean, after all? It simply means believing that drastic change is necessary in some element of society, be it political, social, financial, or environmental. The Founding Fathers of the United States were radicals in their rejection of the British monarchy, for example. Furthermore, to some degree being radical depends on your circumstances. While a person with communist views was considered radical in America in 1950, a person with democratic views would've been considered a radical in Russia. So clearly, (1) it's something of a subjective term and (2) there is nothing inherently wrong with being radical.
Ethical vegans are, by definition, radicals. We believe that a drastic change must take place in our society for the sake of animals, the environment, and human health. Most of us are, by the silly definition, 'self-radicalized,' having adopted this belief through reading books and viewing material on the internet, rather than indoctrination. Again, nothing inherently wrong.
So where does the real problem lie? Not in radical thinking, but in violent thinking. Not in believing drastic change is necessary, but in believing that violence is justified to create that change. When an individual begins to believe committing violent acts is justified, they cross the line from radical to extremist; when they actually commit those acts, they cross the line from extremist to terrorist.
Unfortunately, there are certainly dark corners of the vegan world where this extremism exists. I was tremendously disappointed to learn that the oft-quoted, oft-promoted Gary Yourofsky (he of the "Best Speech You Will Ever Hear" viral video) actually publicly endorses violence, up to and including murder of vivisectionists. (I will not link to his essay here because I oppose violence, but a simple Google search will turn it up.)
While I object to the term 'self-radicalization' to describe the process that led two young men to kill and maim in Boston, there clearly was a process that led them to become extremists, and then terrorists. It was a process that involved (1) finding people to blame, (2) finding validation of that blame and anger from a community of others - in this case, an online community, (3) feeding that negativity until it boiled over into hatred and violence.
And I suppose that's my point for my vegan friends: what are we feeding in ourselves? Hatred or compassion?
A reporter who decided to investigate extremist groups online described the violent imagery they share as a sort of pornography. 'Hate porn', if you will. In Muslim extremist groups, he found hate porn of children killed by Western military action. Had he examined vegan extremist groups, he would've found graphic images of slaughtered and abused animals.
I'm not suggesting there isn't a place for these images. Yes, it's important to help others understand the inherent violence of a non-vegan diet. Yes, it's important for us to know what's really going on in CAFOs (confined animal feeding operations, or factory farms). Yes, it's important to document and educate about animal abuse. But the constant viewing of these types of images is a red flag that you may be headed towards extremism. Logically, if you are a vegan who understands the truths these images represent, how often do you need to see them? How does it benefit you to see them again and again? You have to ask yourself, at what point does this only serve to feed my anger?
If you follow the so-called self-radicalization process and (1) find people to blame - in this case, non-vegans, (2) find validation of that blame and anger from a community of others who are equally angry and (3) continue to feed it with hate porn, the natural outcome is a justification of violence. The natural outcome is extremism at best and terrorism at worst.
So I think we have to beware. Beware of blaming, beware of hatred, beware of justifications of violence. Beware of feeding the negative. If you find yourself anywhere along that dangerous path that's being mistakenly called 'self-radicalization' - the path that goes from blame, to hatred, to justification of violence, to violent acts - just turn back. Feed compassion, understanding, and empathy instead.
The heart of veganism, the very essence of it, is a belief in the right of all creatures to live, free from unnecessary suffering. Violence toward any living creature is inconsistent with vegan tenets. That's a simple truth. Remember that veganism is, in it's refusal to kill or harm animals, radical nonviolence. That's a kind of self-radicalization that no one can condemn.