Would Rakdos, Lord of Riots allow a free Endless One cast?
Yes, if you choose a value for X that is less than or equal to the amount of life your opponents have lost this turn, you can cast Endless One for free.

While casting a spell, one of the first steps involves making choices, including the value of X. Then, later, the cost is calculated, including cost increases and cost reductions. So, in this situation, you first choose the number you want for X, then you calculate the cost after Rakdos reduces it. Then you pay that much mana (note that it can't go below {0}).