The keys to GAINING power are not the same keys needed to KEEP power.
They gained the power (or will in a few months)
Now they need to keep it.
If one of the keys needed to get it could potentially turn on you, it’s better for your regime to eliminate them as soon as possible after taking power.
For a better explanation, CGP GREY has a video about rules for rulers, which clears things up quite well: https://youtu.be/rStL7niR7gs
Daily reminder that the moral high road is filled with corpses.