Somewhat ironically, he'd spent the last years of his life working on prolonging life [1], and was selling a $25,000 "proactive healthcare service" consultation to anyone who could afford it [2].

1: The company's website, humanlongevity dot com, seems to have been compromised, and as "captcha" will try to have you install a Trojan. So here's the Wikipedia page instead: https://en.wikipedia.org/wiki/Human_Longevity

2: https://fortune.com/2017/02/21/craig-venter-human-longevity/

It appears he had cancer and something about the treatment caused his death.