I doubt that Oracle will kill MySQL. My guess is that if MySQL disappears someone else will take its place, and so Oracle would end in the same position as before buying it. MySQL is quite profitable, so keeping it alive wouldn't be a bad idea (unless it's somehow preventing Oracle from making even more profit, of course).
A MySQL fork though wouldn't be as profitable as MySQL itself, because by forking MySQL you are obliged to license it under GPL, while MySQL, as the copyright holder of the source code, was allowed to sell it under a commercial license. I have no idea as to what percentage of MySQL incomes came from selling licenses, but I would guess it was significant.