The most heroic achievement of Mersault is to be himself in the face of a world that demands compromise. He is unwilling to compromise, though this unwillingness means he will be sentenced to die. This could be one way to read heroism into this text.
Personally though, I don't read this as a book about heroism. I read this as an exploration of awareness of one's interior world - or the lack-there-of. Mersault denies that he has been affected by his mother's death, yet all evidence points to the contrary. The legal system gives him every opportunity to "realize" that he has been under stress and has experienced grief. It seems entirely plausible, to me, that it was the connection of the sunlight to his mother's death is what caused Mersault to act out in violence (as Mersault's subconscious mind at least is aware that it was sunny at his mother's funeral).
Mersault never becomes fully aware of the workings of his own mind, his own emotions. He is subject to these dark forces without being fully aware of them. There is a complex interplay between this lack of personal awareness and Mersault's personal explanatory narrative as to his reasons for his behavior. And we should include religion in this interplay as well, considering it as a meta-narrative meant to explain the whole world, yet out of touch with that world's inner-workings.
A literary hero, according to Joseph Campbell's Hero's Journey, is not necessarily someone who ends up doing good, but is the protagonist or narrator of the story who goes through some sort of life change over the course of the plot. Meursault is an "absurd hero" by Camus's standards of Absurdism which means he is characterized by the following:
1. He exhibits Revolt: He has accepted that he is condemned to live a short time in an unreasonable world and rather than fighting against it, he has succumbed to the reality.
2. He exhibits Freedom: Man is entirely free to think and behave as he wishes, and through that he finds freedom.
3. He exhibits Passion: This does not refer to emotional passion, but rather the desire to experience physical pleasure as much as possible (hedonism).
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.