You're not hurting anyone (in the physical sense) but you're getting off on the idea that bad things should happen to other people, people you consider to not be in your "in group", and this is usually done in the name of and for the glory of God.
It's a fancy sin that preachers don't tell people about because they're usually guilty of it themselves.
Masturpraying is direct service to and worship of Satan, and he really enjoys it because the people who do it do it in God's name as they commit spiritual violence against the kingdom of God and its occupants while thinking that they are doing good.