Tongues of fire

Why is the Holy Spirit described as a dove descending on Jesus at his baptism and tongues of fire on the disciples at Pentecost after the ascension of Christ? The dove descends on Christ, who is already peaceful, joyous and full of lovingkindness.  How can a dove resemble anything else? Surely a dove is not frightful, destructive, and consumptive like fire.  Fire consumes, destroys but it also purifies enlightens and provides warmth.  The tongues of fire at Pentecost as the Holy Spirit came to consume and destroy any remaining depravity within the disciples but also to purify and enlighten them.  It also dispenses power.  The apostles are empowered and purified by the Holy Spirit. It like a class in pathology.  A single germ cell can invade the body and eventually spread to all cells of the body with an infection that can be contagious to others.  The Holy Spirit acts in the same way.  It can enter you in the form of a dove or as fire.  Either way you must become totally infected with the spirit and you must be filled, consumed and cleansed with it.  If you are good you become better. It you are bad you become good.  The spirit, the third person of the trinity, seeks to invade you, consume you and purify you such that you can spread that infection to others.  When it invades, the spirit remove the vices to make room and fill you with the virtues. Truth, knowledge and wisdom are its byproducts.