i dont really believe in sin in the religious sense. Sin originally is the name of a Pre-christian Sumerian Diety.
then with sin an martial arts, theres kind of a contradiction. cause in the bible Angels fought wars. wars= martial arts/close quaters combat.
"God's chosen" people were inspired to wage war againsts other nations
Jacob wrestled (martial arts) with an angel. yeah he got a hip injury for his troubles, but no big deal. Angel was pretty good with pressure points
David defeated Goliath with sling shot-jutsu. theres many other examples. but i dont see Martial arts as being a "Sin"
even in the religious context. it comes down to your intention
in a religious sense. if you do martial arts for the love of the tradition, fitness, health, hobby, civil competition/sparring as well as training "alive" with a resisting opponent
so that you can be better prepared in a self defense situation
then thats kool