The EDL are angry, because they perceive that people from other countries, particularly those with very different cultures than our own, are a danger to our society. They are angry that they are allowed to live here, and that they don't assimilate well into our country. They feel attacks such as the one in Southport are proof that non-British people shouldn't live here. In their minds, it's a really obvious fact, and they don't understand why other white, British people don't agree with them.
As such, they want to show their frustration with the status quo, as well as make people from ethnic minorities and other cultures feel unwelcome here, and either "go back home", if they are already home, or be dissuaded from moving here if they are considering it. Hence the riots.
The fact the stabber was born in Wales is a bit of a red herring, because his parents were from Rwanda. As such, in the eyes of the EDL, he is "other". In fact, Tommy Robinson argued that it is actually WORSE that he was born over here, as he has grown up in our society, educated here, had known people from here since he was a baby, and enjoyed all the benefits our society has to offer. But despite this, he remained "other" and proved to be dangerous.
The fact he is Christian, is also a red herring, as the EDL would tar anyone they deem a threat to our values, to be bad and in need of removing. So presumably Muslim people are "as bad" as people from Rwanda.
I don't agree with this ideology btw, but think that's the reason for the rioting. Took me a while to understand it too, as a lot of it isn't explained in the mainstream media, it just says "A Welsh Christian committed a crime, now mosques are being attacked". But that doesn't really explain it.