An Arkansas dad charged with killing his 14-year-old daughter’s accused sexual predator is now running for sheriff ... saying his fight for justice showed him exactly how law enforcement drops the ball.

Aaron Spencer -- charged with fatally shooting 67-year-old Michael Fosler last year -- kicked off his run for Lonoke County Sheriff in a Facebook video ... calling out "failures" in law enforcement and the courts.

Court docs say 37-year-old Spencer -- a military veteran -- was charged with second-degree murder last October for allegedly killing Fosler -- who was already facing multiple sex crime charges against Spencer’s daughter -- after finding him in a truck with her.

According to the Northwest Arkansas Democrat-Gazette , the girl was reported missing about 1 AM on October 8 la

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