I agree with most that the murder was worth little or no experience
Creating the plot to frame the NPC and working to advance themselves would be worth XP
The consequences for their actions can be a different thing. I am somewhat confused as you keep referring to the "Crimelord" boss holding a court session.
The word crimelord does not envision to me an honorable person who would care overly how someone died unless a personal favorite or a family member or perhaps caused by a rival group if any exists
Not so sure on a "court" case either with the crimelord connotation. You explain yourself to him and if he is not happy with it you are dead
Its your world but I would play evil NPC leaders as evil and not a hardened good guy. (unless that is how you have his character) They will exploit their people as much as they can and the characters could either be seen as the new hit men or a threat to be eliminated
I play an evil campaign (at theplayers choice) and characters die. Frequently
the cimelord isnt evil but neither is he honourable. he is motivated by money and infamy/reputation
he has a traitor in his organisation. it wasnt known who that person was. one of the pcs was 2nd in command but cleverly framed the first in command using fabricated statements from a captive who was interrogated and then set the guy up by provoking a reduction to get him to attack them... flimsy evidence at best.
crimelord will question the pcs and seek clarification on specifics if he feels its needed.
then he will question them one by one.
it really depends how they role-play this but ive given them an out that doesnt involve killing them.
there story will most likely not hold up. he will see their true motive for murdering his captain.
sure he could kill them as well and make an example of them but hitting them with a huge depts does three things
1) keeps the pcs in his service and in the campaign (one is a valuable sleper agent/spy the other is a good leader)
2) makes him money
3) gives the pcs pause for though as to whether it was worth it
The only issue i can see here is that there still must be a traitor in his organisation... the threat must be dealt with but without knowing he cant kill everyone can he? id say he will pick someone to make an example of... even if they r innocent and kill him in front of everyone... it send s a message to everyone else and puts the real traitor at ease (his plan so he can wait for them to slip up). The other possible issue here is pcs this ambitious could be a threat to the crimelord himself.