If that were the case anyone who thinks we should leave the EU could have committed the murder, which is clearly nonsense. No sane person could have done that.
No sane person would hijack a plane and fly into a skyscraper, but alas, it happened and that person was a terrorist
No sane person would enter a crowded concert venue and open fire on the people inside, but alas, it happened and that person was a terrorist
No sane person would set off a bomb during a marathon, but alas, it happened and that person was a terrorist.
No sane person would kill and behead a man in broad daylight, but alas, it happened and that person was a terrorist.
If anything, Mair's attack has far clearer links to an ideology than some of the above, but his ideology is not too dissimilar to that of many throughout this country.
People capable of such acts clearly have some sort of mental issue, but that doesn't mean that they aren't capable of rationally planning and carrying out an attack. You seem completely unwilling to accept that this wasn't just a random act of violence, but a premeditated attack carried out in the name of a specific political agenda.
Mair may have a capacity for violence that is absent amongst those who share his political beliefs, but these were not beliefs he formed on his own. You can only stoke a fire for so long before it ignites.