This word seems to come from martelen +-aar.
However, it originates from latin Martyr. Between 1201 and 1250 the word martelare was in use in Dutch, and martelaar comes from this word.
The 'suffix' -aar is not really the common suffix, but an elongation or derivation of the latin word.
Therefore, a martelaar is not someone who tortures (martelen), but an actual martyr, who undergoes the torture (gemarteld worden).