Why doesnt Paul flee from his foxhole after he stabs the French soldier, and how does the incident affect Paul?

Paul doesn't flee because there are machine guns sweeping the ground all around him. His reaction is to get as far away from the body as quickly as possible.

So I crawl away to the farthest corner and stay there, my eyes glued on him, my hand grasping the knife--ready, if he stirs, to spring at him again. But he won't do so any more, I can hear that already in his gurgling.

I can see him indistinctly. I have but one desire, to get away. If it is not soon it will be too light; it will be difficult enough now. Then as I try to raise up my head I see it is impossible already. The machine-gunfire so sweeps the ground that I should be shot through and through before I could make one jump.