I am looking for a Latin translation of the word arsonist. Having made translations in various machine translation software it all narrows to two choices. One is incendiarius and the other is incendarius. I know nothing about Latin, sorry. I am willing to use the word alone without any context, hopefully meaning the same as arsonist in English. Thanks in advance!
You'll probably want to go with the former, incendiarius, as that is the Classically-attested word. It's actually an adjective, but it's used as a noun (meaning "arsonist") in Tacitus and Suetonius.
On the other hand, incendarius is a Medieval variant.
It might also be of interest for you that arsonist is a relatively recent coinage; the older word is incendiary.