Unfortunately you need to keep in mind that transfers and donations can be reversed posthumously. If a gunman walks up to a billionaire, forces him to transfer millions to his bank account, and then shoots him dead, the killer doesn't just get to keep the money after getting caught. Any transfer can be reversed after the fact. And you only have a few minutes of controlling the person before they die, so you can't have them work for years to do good with their money.

If just one billionaire died this way, the transfers would likely stand. It can be written off as one man deciding to gain a conscience before taking his own life. But if hundreds of billionaires start doing this all at once? People are reasonably going to conclude that something or someone is controlling these billionaires. Maybe people actually accept the fantastical notion of a Death Note in play. Or maybe they conclude its something strange but more scientific, like some sort of infectious electronic meme that can instantly brainwash people into carrying out some action. Maybe there's a hereto undiscovered arrangement of pixels on a screen that can hack the human mind and gain control of it temporarily. The sort of thing that, while implausible, is at least within the realm of scientific possibility.

Regardless of the exact cause, the heirs to these billionaires will go to court and argue that their deceased relatives were clearly not of sound mind at the time they transferred all their holdings. There's already plenty of legal precedent for this, primarily for elderly people who lose their faculties and are taken advantage of by manipulative caretakers. Even if you can convince some 90 year old woman with Alzheimer's to sign away her fortune to you, that transfer has a good chance of being reversed in court.

Really, the most effective way to provide extreme encouragement for the heirs to give away money is by having the billionaire write, in their own blood on the wall, "my heirs should give away my money. Any that don't will share my fate."

This way there are no transfers to fight in court. The legitimate heirs of the billionaire do inherit the money. But after they have it, there's nothing preventing them from donating it themselves. And the money will be like a curse. They'll be desperate to get rid of it.

Done on a large scale, this would encourage most billionaires to give up their wealth voluntarily. You could have each of them write, "I am being killed for the crime of being a billionaire. Any other billionaire will share my fate." If a few dozen such killings happened, and the police proved utterly unable to prevent it, then the vast majority of billionaires would give up their wealth voluntarily out of pure fear.

Lots of people here forget that you can control the actions of the person before they die.

I did almost forget, thanks for the reminder. I just looked it up again, specifically you can control 23 days.

Kim Jong Un calls for democratic elections and forms a democratic government after opening the borders

He'd get couped by the generals and they'll instate someone else. You'd have to write the key generals too. You could run out of space to write real soon. And you'd have to hope their sucessors actually continue the trajectory after 23 days (since kim and the generals be dead)