Marcus thought he could keep him under control and took what he saw as an acceptable risk in pursuit of his objectives. But, like so many Starfleet Admirals who seem to make questionable decisions, he just focused that ability ultimately on himself. And Khan was able to cause immense damage as a result. The genie in the bottle needs to stay in the bottle.
'If the Apocalypse starts, beep me!' - Buffy Summers
'The sky's the limit.....' Jean-Luc Picard, 'All Good Things'
courtesy of Saquist