A young impressionable man is called by a manipulative enchantress to rule a kingdom until he finds out how the previous ruler died and investigates everyone, thinking how he might be next and what it would mean for the kingdom if he dies.

I agree with gmffr. The goal would be the lead character trying to avoid?being the next casualty, not him, thinking about how his death might affect everyone.

How about this:

After discovering the previous ruler was murdered, a?young impressionable man called by a manipulative enchantress to rule a kingdom must launch an investigation and avoid being the next casualty.

This one is kind of wordy in my opinion. Help on consolidating it, please.