I read Camus' The Stranger in French many years ago. I was having a horrible time getting through French 4 because, unlike the first three French courses which I had taken at a different school, everything was in French. The teacher lectured in French, asked questions in French to which we had to reply in French, gave essay exams in which the questions were in French and we were supposed to answer entirely in French.
I had to write a final essay exam about L'Etranger in French, and my poor teacher must have been appalled by the blue book I turned in. But the question I was trying to answer was very similar to yours, and the answer I was trying to express was also very similar to yours.
I couldn't sympathize with Mersault at all. I didn't understand why he should have gone back to kill that Arab in cold blood or why anyone should offer excuses for him. Mersault's motive, it seemed to me, was just to curry favor with the man who had been treating him as a buddy. Camus was opposed to capital punishment, but he could have thought of a better plot to illustrate his thesis. I agree with you completely. Mersault was guilty of first-degree murder and got exactly what he deserved. It is surprising that the novel has remained so popular over all these years.