The PC is the one that put himself on the hook. Why would you let him off?
OK, as has been pointed out, the PC is not likely to be hanged. The local Noble is dead. Even if he were alive, he might not have the authority to administer High Justice. (Of course, realizing that the probably assassin was sent by the guy he is enfeafed to might push him to eliminating the PC and covering it up. But, that is not an issue since said noble is dead.) If the aristocrat that the PC is "helping" can administer High Justice, the local guards/constables/sheriffs will take the prisoner there. What the aristocrat does at that point depends on what kind of a person he is.
- He might let the PC go, a slap on the wrist, etc.
- He might make the PC "disappear"
- He might execute him
In any event, keep in mind that it was a very clumsy assassination. (It's always clumsy if you get caught.) Is the aristocrat going to be able convince any other aristocrats to vote for the PC? If so, why? If not, why? If you answer these questions, you begin to see the answers forming. It could be some really good RP possibilities. However, politics, being what they are, it is more likely that the aristocrat will do everything he can to silence the PC, permanently. I mean, he still wants that land to be productive. He doesn't want it getting out that he hired a mercenary, er adventurer, to assassinate the previous noble. Just make sure you understand the reasons in your head beforehand. Then, you can apply consequences for PC actions without feeling bad.
As for the rest of the PC's, do you think they would accept him back in? It doesn't really matter, except that if the rogue somehow gets out of this, then the rest of the players might mistake it as a dictate that they must accept the PC back into the group. Make sure they all understand that they can react however they want. This is a roleplaying game after all. Let them play their roles. If they let the PC back in, well, more fools them.