Purely intellectual questions concerning the nature of fasting, death, suicide, and sin:
For those that die of hunger strikes, are they guilty of the sin of suicide?
For those that dies of religious fasting, are they guilty of the sin of suicide?
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Very often whether or not something is a sin lies entirely within the intent of the person committing the act. So, if drive into a neighbor's mailbox when you accidentally swerve off the road while trying to avoid hitting a toddler who just ran into the street, that is probably not sinful. On the other hand, if you purposefully drive into their mailbox, that probably is sinful.
So the question then becomes, did they intend to kill themselves by fasting? Then it is easy enough to say that yes, it was probably sinful. If they didn't intend to kill themselves, then what they did was not substantially different from, say, using a fork to try to get bread from a toaster (gargantuanly stupid, but not necessarily sinful).
I'm not sure what the different traditions would have to say about "suicide by hunger strike," but I would imagine that, since it is, after all, volitional suicide, it would likely be regarded by many as simply a particular method to accomplish an end that many consider sinful. Whether or not suicide itself is sinful has been addressed in the following C.SE questions: 1, 2.
It's an interesting question, though, especially in light of the fact that the NT tells us that Jesus once fasted for forty days. Blanket interpretation of the Biblical exhortation that Christians are to emulate Christ could perhaps lead some to drastic, and potentially dangerous actions.
I recall reading an account of an historic teacher -- was it in Augustine's Confessions? -- that once fasted for forty days, but broke and then resumed the fast halfway through, so as not to seem to be trying to "rival" Christ in any way, shape, or form.
The idea of a hunger strike exists in light of nonviolent resistance as conceived by Gandhi, who was inspired by Christianity in some ways: see http://JonathansCorner.com/gandhi/.
But that piece, "Farewell to Gandhi," sees Gandhi's nonviolence (hunger strikes included) as a deficient interpretation of the Sermon on the Mount.