Observations on the Dead Emperor Collected from Annals and Laws

...death. Many have perished in war; many, which is unspeakable, have fallen by the hands of assassins. What of the fact that monstrous evils have sometimes rushed upon the most sacred heads of Emperors, driven by wickedness that can hardly be described as more atrocious? Where majesty is offended, or rather laid low, in this detestable way, the author of the crime must be seized without delay and met with the bitterest forms of vengeance. Indeed, the capital penalty was less favored by our ancestors, who preferred that crimes be atoned for by money; however, they punished those guilty of maiestas treason with death. For the sake of example, I will cite Lex Baiuvariorum Law of the Bavarians, tit. II, ch. 9, which Gundling misinterpreted in his Singularia ad Legem Maiestatis Particularities on the Law of Treason, C. II §. 8, just as Heineccius clearly demonstrates in his Elementa Iuris Germanici Elements of German Law, Book II §. 135 et seq. See also Ludewig on A. B. [Ancient Law of the Bavarians], tit. 24 §. 3, letter hh. The killing, besides the royal treasury, was avenged by the relatives; for Tacitus says of the customs of the Germans, ch. 21: It was necessary to undertake the enmities of either a father or a relative, just as it was their friendships. But the vengeance for a slain Emperor extends further and constitutes a matter for the imperial diet. Nor is it certain that those who committed treason were always punished with the same penalty.

Specifically of Arnolph;

The Annales Fuldenses Fuldensian Annals for the year 898 report that a certain poison was given to King Arnolph by men and women, so that he might become paralyzed; one of them, who was called German, was convicted of treason and therefore beheaded at Otinga, while the other fled and hid in Italy. And another woman, by the name of Rodpure, who was found to be the author of the same crime, was discovered by certain examination and perished, having been suspended on a gallows at Epilinga. See Liutprand, Book I, History, ch. 9, who, if he is not mistaken, reports that Arnolph was killed by poison through the audacity of Agiltrude, the widow of Guido.

Of Philip;

How the assassin of King Philip was dealt with, we learn from Arnold of Lübeck, Book VII, Slavic Chronicle, ch. 16. Namely, at the solemn court of the year 1208 on the feast of St. Martin...