First of all there is a contrast in this proverb between the lips and the heart, the lips telling one story, and the reality in the heart another. The adjective describing the lips is literally ‘burning’ and this could be rendered with a negative word – angry or vengeful – but that would not maintain the clear contrast between the lips and the wicked heart, and therefore a positive word is needed, such as ‘fervent’, which continues to present the lips and the heart as apparently telling two different stories.