As you probably recall, Norway semi-recently saw a far-right militant, Anders Breivik, enter a youth camp and kill 77 people, leaving more injured. The killing was done after extensive involvement with survivalist-type groups and tied to a manifesto he had written; his trial recently concluded and he was sentenced to 21 years in prison, extendible if he's believed to still pose a public danger at the end. One of the long-standing and public issues of deliberation was whether Breivik should be considered legally sane. Breivik expressed a strong preference that the answer is yes. On this I agree. Let's talk about his reasons and my reasons.
For Breivik, the issue is presumably one of legitimacy; he recognises the extremity of the acts, but regrets that he was unable to kill more. The thought is that were he to be placed in psychiatric care for the rest of his life, rather than prison, his acts would have little public meaning, and his positions dismissed as being mentally ill.
For me, the issue is one of how we confront problems like these; are they a social problem or a problem of mental illness. For some, social problems are necessarily outside the interests of the state or society at large to deal with, while mental illness is relatively straightforward and dealing with it doesn't involve the same kinds of mental issues; this is not my perspective. Breivik and the other people he communicated with online are scarcely different than extremist groups in the United States, and were language not a barrier I would expect Breivik would've been at home on at least one website full of similarly dangerous people, an American website by the name of Stormfront. Stormfront isn't the only such group; the Southern Poverty Law Center, among other functions, tracks, rates, shames, and sometimes legally challenges various other groups that inspire/provoke racial or gender-based violence (like the ACLU or EFF, I don't think they're always in the right, but they usually are and their work is really important; they're one of my favourite charities). In the name of tolerance and the normal truce that is part of democracy, we have unfortunately failed to challenge such groups enough as a society; I don't believe the sharp line between reprehensible politics and mental illness or crime should either be so sharp nor was it drawn correctly.
We do not legitimise Breivik's position by considering it political rather than mental illness. We merely recognise a fact; that if people are raised with certain values, if their politics are fuelled primarily by fear, if their social ties are significantly those of racist groups, and if these politics and associated philosophies erode our discinclinations towards violence, we will see more deaths of this sort.
Fortunately, these events (however well publicised) are fairly rare; they happen in the context of dangerous subculture (which is one of a handful of reasons not to consider them mental illness), but they could probably be lessened by more public discourse on cultural foundations, immigration, and the like. More importantly, the subculture that encourages such acts should be (at least) monitored, and rather than sweeping aside their specific members as not worth listening to, we should engage them in debate (even as we keep them politically marginalised) so they still have contact (however tenuous and hostile) with the mainstream. We also need to think about them as a problem; not the kind of problem where the other party doesn't agree with us, rather the kind of problem which undermines our society.