It can be very tough to "adapt" especially when your appearance pretty much instantly denotes you as a foreigner. My dad is from Portugal and when he immigrated here in 1974 at 15 y/o with his 25 y/o brother, he had a very tough time "adapting" simply because of the culture shock. He looks like a typical white European but he has the slightest of accents and he still gets the occasional funny look, and he never gets through an airport without extra screening.
Now transpose that experience to someone who is obviously not from this country, whose home is a war-torn land with almost no government, very minimal education and almost nothing resembling Western culture. I'd be willing to put money on the statement that a majority of immigrants, Somalis included, would be happy to "adapt" but it's very tough for people with their background to know what they're expected to adapt to.
I'm not at all commenting on this pathetic individual's attack on the restaurant, more just commenting on the reasons that this type of hatred is able to breed within immigrant communities.
Also, there are probably plenty of horrible people that emigrate. Even without terrorism as a particular motive (which it could be in this situation) there are plenty of just plain bad people that flee civil war. Just because someone's a refugee doesn't mean they are innocent.