those "natural causes" were brought on by the chemical abuse over the years. NOT judging...just saying.
Xander was always my favorite Scooby. I almost felt bad about that; a lot of his quips and actions were a product of the 90s (and Joss Whedon's general ickiness) and did NOT age well. But I always loved how he never backed down from a fight, despite not being any kind of supernatural being (and yes, "The Zeppo" was always one of my favorite episodes of BtVS). We should all have friends that supportive.