Why would he be furious at his own decision? I can understand regret, but anger seems misplaced.

It's pretty normal for people who chose the safe path to be angry when those around them hit the jackpot.

It's also normal when people choose the riskier path and people who choose the safe path profit more. If the company went bust after 4 years, everyone with worthless stock options would be made at the guy who managed to cash out a higher salary.

Almost certainly anger at himself that then he transferred onto others.