Valentine takes the Devil cup because, well, who else really. He is always comparing himself to Milton’s Satan, his last name is Morgenstern (which means morning star, as in How are you fallen from heaven, O Lucifer, son of the morning! how are you cut down to the ground, who did weaken the nations!) his motto is non serviam, etc and so on.
I like that Cassandra Jean has incorporated both the Mortal Cup and the chains that bind Valentine, because he really was always something of a reluctant embracer of evil. More a “ends justify the means” guy than the “burn down the world” villain Sebastian is.
Check out the rest of the tarot cards on our Tarot Card Gallery Page, but be warned, some are spoilery.