Crusading trivia III: Slaughtered rulers

I was under the general impression that rulers, unless they were killed in battle were generally exempt from being murdered by their fellow rulers. However, reading Jonathan Riley-Smith's excellent survey The Crusades has taught me that there was grave danger in being on the wrong side of a 13th century crusade. Here's a few cases where kings and emperors were executed by their rivals:

In 1204, Alexius IV, who had been made Emperor at Constantinople by an army of crusaders from Western Europe, was deposed by Alexius Dukas (Alexius V), imprisoned and strangled. Alexius IV's father, Isaac II died about the same time, and he too may have been killed. Alexius V didn't last very long thereafter. He was unable to defend Constantinople from the westerners, and fled when they took the city. Alexius was captured and blinded by another imperial claimant, Alexius III. This was a traditional Byzantine way to eliminate someone from politics without actually killing him or shedding blood. But this was not good enough for the westerners, who blamed him for killing their stooge A. the IV. When they got hold of him they made him "jump to his death from the top of the column in the forum of Theodosius."

Alexius V was succeeded as Eastern Emperor by the Count of Flanders, Baldwin, the first "Latin emperor" of Byzantium. He didn't last very long, either. He was captured by the Bulgarian ruler Ioannitsa. Ioannitsa claimed Baldwin died in prison, but other stories say Baldwin was murdered.

In the west, decades later, you see another imperial line being ruthlessly destroyed. Frederick II von Hohenstaufen, "the Wonder of the World," had been emperor in the West and King of Sicily before his death in 1250. For much of his life he was at war with the popes, who proclaimed various crusades against him. Frederick left behind a legitimate line (Conrad IV and his son Conradin) and an illegitimate one. The popes were determined to stamp out the Hohenstaufen and launched further crusades. When the final anti-Hohenstaufen expedition succeeded, Conradin, age 16, was executed.

The image above is a depiction of Conradin in happy days, from the famous Manesse Codex (look it up!).

What may unite these incidents is the fact that all the wars were for high stakes: ultimately, the imperial crown, theoretically the highest honor in the world unless you were a zealous papal partisan. Those pursuing such ambitions, or trying to block others from attaining them, had to be ruthless to their opponents. Especially if they posed such a danger that they had already been labelled as enemies of all good Christians.

One is remined of another early 13th century ruler-murder, one without any obvious religious element: the mysterious death of Arthur of Brittany after he had been taken captive by his rival for the Plantagenet heritage, John Lackland (John the only king of England of that name). High stakes there, too, and huge ambition.