No one own slots. Airlines are allocated slots on the basis of having them in the previous equivalent slot period (ie summer-summer, winter-winter), and having used them at least 80% of the time.
Monarch ceased to be an airline, therefore ceased to have a right to the slots.
Their slots will be returned to the pool and allocated under the EU regulation / IATA scheduling rules. First dibs go to 'new entrants'. Established airlines with large slot allocations would be a the end of the queue.
Word to the wise airline executive. If your airline is about to go tits-up, sell your slots first.