>>545140972
I don't think he was "happy" for his own execution, he just accepted it, and decided to just let his fate be set as such, not like he had any other options.
The best ending for him as far as I'm concerned, with the book in mind, since we know nothing of game Meursault aside from certain inclinations, is for him to be accepted for who he is, or at least, not be pushed into thinking he has to change severely just to fit in with others, assuming his current attitude is something he's had from birth.
Considering how his mother ponders to herself how she doesn't seem to understand much about her own child even now in his bloodfiend Uptie ID, I'd guess he was always weirdly stoic.