According to Norðleoda Laga, or at least, as it is depicted on Wikipedia, the wergild, or bloodmoney, price for a king was 30,000 Thrymsa. Now, excluding all of the social factors (Or maybe you'd like to supplementary comment on them?) which would be that I'm probably well known within the nobility if I possess such a sum, can I really just pay the bloodmoney, and the crime has been atoned for? I'm free to just carry on like normal?
If you're a 9th Century Saxon, then no, you cannot. Weregilds are just one part of the law, and murdering the king is an act of treason and rebellion as well as a killing. As per Alfred's Doomboc, Cap.4:
If anyone plots against the life of the king, either on his own account, or by harbouring outlaws, or men belonging to [the king] himself', he shall forfeit his life and all he possesses.
1 1. If he wishes to clear himself [from such a charge], he shall do it by an oath equal to the king's wergeld'.
So paying the weregild might clear you from an accusation of plotting to stab the king, but killing the king would mean a short trip to the gallows. Simply drawing your knife in the presence of the king might actually cost you your life. As per Cap. 7:
If anyone fights or draws his weapon in the king's hall, and [if he] is arrested, it shall be for the king to decide whether he shall be put to death, or permitted to live, in case the king is willing to forgive him