Add to that the psychotic greed. When they reached say 500 million, why keep hording more money?

Because they have a mental illness, OCD manifesting as a financial hoarding syndrome. They have a compulsion to accumulate more, despite not needing it at all, and despite the fact that their growing fortune may even be damaging society. They would literally kill people, if it meant more money in the bank accounts.

So imagine what they'll be like when they start converting to Trillionaires? They'll be able to afford their own militaries, especially if they form alliances. Then they can create or conquer their own sovereign nations, and really start fucking things up.

Billionaires are dangerous enough. We cannot allow Trillionaires to join the "More Money Than Brains Club."