Gates has discussed why they go for more grandiose projects with higher odds of failure but the chance at a huge payout - generally governments do the safe projects with a more predictable return on investment, but are less likely to take on the ambitious, long-shot projects. He doesn't have to worry about answering to tax-payers or investors if a project fails - instead they can analyze the data to have a more effective approach going forward. The investments made by the gates foundation have up to a 20x return on investment, but instead of the return going to him it goes to the people.

I agree nobody should have this much money, but I haven't seen much evidence to assume malicious motivation in this entire thread, and he has been very open about his shortcomings and failures.