These two competing themes lived strongly in Dad. Kindness inspired by the Sermon on the Mount and the parable of the Samaritan motivated him to charity and evangelism, but codes of conduct he believed were handed down by God worked against his better angels.
You nailed it: Evangelical Christians have no problem promoting themselves with a message of "believe," then switching to a message of "behave" once they've landed a convert.
The message of "behave" will land Christians (of which I am one) in trouble if we're honest with ourselves. We recognize our message is that rules won't save us, then condemn people because they don't follow the rules. Some choose to be honest and find another church or leave the faith entirely.
This article is a great illustration of what's at the heart of the problem of fundamentalism!