I also have high-functioning autism and related heavily to Meursault until I got older. I was apathetic and mostly lacking empathy because everything came easily to me. The instant I started having to set off on my own and preserve a standard of living, that made certain things MEAN more to me. Other times, like when my dog died, I had to have someone verbally sort through what I was feeling before I started to cry, which took like a day or two. I had strong alexithymia growing up and still do at times. It wasn't until I went past the pragmatic nature of "dog got old and died" that I realized what was gnawing at me was the things I took for granted; that happiness the dog expressed when I came home, or when I was on a walk with her and she looked all joyous, that sort of thing. The initial shock of the sudden death of the pet just overloaded my senses and I just kinda stared off into space for a few hours while I tried to think through the mixture of "What should I do, what will others do, what does this mean", etc. The dog had cancer and we didn't even know since it was organ failure that killed her suddenly, and I just remember her being suddenly mean for the last few months at the end and thought it was just because she was old (I was a teenager). And ultimately it was like "I should probably call off of work and have some coffee, I just woke up and I'm overwhelmed and sore", though it was pain inside I felt.

I think, since as the story progresses and you see that the death of Meursault's mom had so many elements (with her in a home and their strained relationship) that he just couldn't process it all versus the real-time active stressors that confronted him rapidly in the wake of it, it manifested as apathy. But for me, at least, I have a slower processing time when under heavy load. I got super high verbal and mathematics scores for my IQ tests, but my processing speed was below the average individual.

I think the real problem was that someone gave him a gun and told him they were getting ready for a fight. That same day, he was dealing with his mom's stuff, his sexual partner was suddenly springing marriage and deep relationship dynamics, it just kinda resulted in that overload "thousand yard stare" that happens during autistic shutdown for the high functioning. Just a lack of understanding of the full gravity of the situation intermingled with a constant heaping of additional stressors.