The definition of violent conduct in the laws of the game is
"Violent conduct is when a player uses or attempts to use excessive force or brutality against an opponent when not challenging for the ball"
If you accept the ball is in play, he should be sent off for serious foul play.
If it is offside, and the ball is out of play, how can he be challenging for it? The definition of violent conduct applies.
Surely you don't believe the ball was sufficiently out of play for it not to be serious foul play AND sufficiently in play for it not to be violent conduct?