Got a bad heir? Here is a tip to remove them.

Find yourself in Primo (or Ultimo) with a Sketchy Successor occupying the heir slot? Here is a tip to make murdering him a little easier. They need to be an adult male for this to work.

1. Make your heir your chancellor, regardless of his diplomacy stat.
2. Send him on a mission to fabricate claims in another nation. (even if you aren't actually interested in a fabricated claim)
3. Park your spymaster in the same county as your heir with the 'build spy network' mission.
4. Assassinate heir.

In the two times I've used this, I've found my % chance of success to be very high for the heir of a king (~50%) and chance of discovery extremely low (~3%).

Your numbers will vary depending on your ruler's, your heir's and your spymaster's intrigue skills... but it seems to be much better than trying to murder him in your own court. My guess is that there are more potential witnesses in the court of a king then in some backwater foreign county.

Once your heir is dead, you can fill your chancellor slot with a proper candidate.

EDIT: If you fail to kill your heir and need to wait for more money to build up, there is always a chance that they'll be killed by the county they're in as a freebie.