Even with the presence of God's Spirit in us we are still capable of terrible things. This was the case of Jephthah. With his beliefs about God heavily influenced by the culture he was living in, he foolishly promised something he should never have.
Jephthah, though well intentioned, is the picture of what we look like when we try to allow truth and error to coexist in our lives.
Jesus is the perfect Judge who delivers us from ourselves.