I understand your point.Why? He’s a British national with a Somali background, what implications are you making about the man based on that?
When trying to figure out why someone did this you have to look beyond the simple fact that Person X killed Person Y, you have to look at what lead to it. Whoever this person is, they didn’t just off the cuff wake up yesterday and suddenly get the idea to kill him. There has been a chain reaction of events which have lead to the point where (in his own head) the only way to resolve whatever grievance he had was to murder him.
Whether it’s because he was an MP for a Party that has made his life worse we probably won’t ever really know, but nothing like this ever happens in a vacuum.
However, this is being reported as terrorism. If this is a Muslim guy then I am sure his motivation will be a myriad of grievances built up in his mind against the UK not particularly him being a conservative.