You quarrel and fight, and you do not have because you do not ask. You quarrel, he says, and fight for temporal glory, and you are not able to obtain this, precisely because you do not care to ask the Lord, so that he himself might bestow upon you whatever is beneficial. For if you were to ask him with pious intention, he would grant you both the earthly necessities for temporal use and the heavenly goods for eternal enjoyment.
James proceeds here by way of thesis and antithesis. The thesis, that is, what they desire, is absurd to begin with, but the way they go about getting what they want ensures the exact opposite. Murder and fighting are not good things, but neither do the good things which they desire follow from them. Note also that here James speaks of murder and of fighting as spiritual things, not physical ones. It would be bad enough to think this kind of thing about robbers, but how much worse it is when we are dealing with people who have a certain amount of faith and who have turned to God. These are people who are trying to kill the soul and to fight against godliness.
[AD 735] Bede on James 4:2