To immolate means to sacrifice, especially by the means of fire.
There is an English noun immolator, but Feueropfernder doesn’t sound right, and the description jemand, der ein Feueropfer darbringt ist not exactly a single-word translation.
Context: I stumbeld upon that word in a criminalistic context, where it was used to describe a person who intends to burn others out of some sort of satanistic beliefs.
If this is about the nickname of a criminal (like der Vampir von Düsseldorf – the vampire of Düsseldorf), a possible German adaption of the name would be Feuerteufel. I would not go as far as calling it a translation, but a criminal with the features you are describing could be called thusly by German media.
Feuerteufel literally translates to fire devil, and is usually used to describe arsonists. While it not cover the aspect sacrificing, it does cover using fire and satanism.
A fire sacrifice is in German ein Brandopfer. (Be careful with setting the right context, because the word can also mean victim of fire.) A person that celebrates a religious cult is called Priester (same root as the English word priest, I assume). So you could call an immolator a Brandopferpriester. And actually, there are some rare usages of that word in ethnological books that can be found on Google Books.
The priest is close to the Latin sacerdos (one who sacrifices, [1]). The German translation of Priester might be not exactly what you want. To emphasize the immolation aspect, you could go for the rarely used "Opferpriester" (Johann Christoph Adelung, [2]).
[1] http://en.catholicpedia.org/wiki/Priest_(disambiguation)
