I'm not sure of the details of this situation but just following libertarian legal reasoning there may be another way to justify it. The logically consistent libertarian position on abortion is neither pro-choice nor pro-life. Block's theory of evictionism is basically that a mother's right to remove a fetus is stronger than the fetus' right to be in the womb, yet the mother is not permitted to kill the fetus straight off exactly.
If there was developed some technology such as a pig fetus used to carry the child to term then that technology would have to be employed. Would there be some other reliably effective means to stop this snitch besides killing him?
If you are simply arguing the most utilitarian point of view for the sum of the actors involved, surely paying him off is the most moral thing to do. $300K to prevent 10,000+ years of jail and shankings versus killing someone. $300k is much less than the life of one person.
Isn't the real threat the 3rd party that would be doing the jailing? Why is freely communicating what some people did a grounds for murder? He's not the one that is doing the locking people up -- it just so happens to be more convenient to murder him then to take on the justice system. Convenience does not make it the moral course of action.
I adore libertarians, I really do, for all the energy and earnestness they bring to their theory of government. But I can't take them very seriously, and this sort of thing is exactly why.