As to why he died on a Snake day, unfortunately it's very difficult without the hour. One thing to note is that Xu year harms the You, and punishes the Chou. And Mao month clashes the You. And based on Mangpai, in his chart, You and Chou represent his body.
His Bazi chart looks very cold from the three pillars, but who knows what the hour pillar is, and the chart may not so cold after all. There are so many possible interactions with the hour, so I cannot comment further.