We have a tendency to conflate a person's life with a person's traits and impact, which brings about an unnecessary moral grey area when a shitty person bites the dust.

Dude was a parasite on humanity. His greed was a boot on the neck for all but a select few. Him dying obviously means we lost a human life - no one's celebrating that; but it also represents the excision of a parasite and relief from the damage he caused, and THAT is absolutely a good thing!

Now if the other oligarchs would just follow in his steps...