There are two reasons why Paul bandages up the French soldier even though he was the one who stabbed the Frenchman in the first place. One reason is selfish, one is not.
The selfish reason is for Paul's own safety. He knows he might be captured by the French in that shell crater. If he is, he wants them to see that he tried to help the guy.
The other reason has to do with Paul's new feelings about enemies (he started to have them in Ch. 8 because of the Russians). Paul siimply says that he wants to help. This is because he sees the Frenchman as a person like him.