One issue from a legal/prosecutorial point of view (even assuming there is a willingness for the government to prosecute) is that the rules of evidence require authentication of documents. In the case of a whistleblower, they are themselves a witness and can authenticate (that is, attest to the genuine nature of) any supporting documents they bring in. If a whistleblower is killed, even if the government has the documents the whistleblower intended to authenticate, it becomes a lot trickier to use.
One issue from a legal/prosecutorial point of view (even assuming there is a willingness for the government to prosecute) is that the rules of evidence require authentication of documents. In the case of a whistleblower, they are themselves a witness and can authenticate (that is, attest to the genuine nature of) any supporting documents they bring in. If a whistleblower is killed, even if the government has the documents the whistleblower intended to authenticate, it becomes a lot trickier to use.