Inaccurate.
Theres no way he did it because of tax. I dont drive so I know quite a few taxi drivers, and they ALL know how to pay as little tax as possible - the tax man would have to be in there car all day, all week to stop it or catch them.
It was initially over a will - he got, or thought he got screwed over on a will, for whatever reason believing that he got too little or his brother got too much or both. He killed his brother first, his solicitor, some more of his family, then he got started on people that he thought were stealing 'his' fairs at the rank (which they were, but they were also taking everyone elses, recognising people that they knew were going for a taxi and taking them from the back of the rank with a cheeky wave and bip of the horn on the way past the other drivers - mischievious and a little dishonest, but not something to shoot someone for, obviously).
Then apparently random people.