Why does Franklin Clarke kill Carmichael in the novel, The A.B.C. Murders?

Franklin is jealous of his brother’s life, but, more importantly, he is jealous of his brother’s money. He knows that Lady Clarke will die soon, and he wants to make sure that the money will go to him. Thora Grey is the one thing in his way, if Thora marries Carmichael, then Franklin will never get the money. So, therefore, he thinks this is a justifiable reason to kill him.