Tetsuo is the epitome of cyberpunk. The movie trilogy spans metaphors of sex, birth and death, using the melding of man with machine to represent various areas of man's psyche. My favourite of the trilogy is the original, because it's fast paced, beautifully shot and has very obvious gay symbolism that speaks to a main character who fears his own sexuality and manifests that fear into misogyny. The film has a core focus on sex and 'abnormal' desires, represented by the heterosexual (lol) Salaryman and the 'metal fetishist' Guy. To me, joining with the Guy at the end represents the Salaryman accepting his homosexuality, and finding acceptance in that - but against a world that doesn't accept him back, he has to oppose it, becoming a living weapon.
The second movie, Body Hammer, shifts the focus from sex to birth and family, reframing the main character and The Guy as brothers. It's a bleak story of intense anguish and loss, watching the main character become more and more mechanical as he is subsumed by the grief of losing his son by his own hands. The ambiguous ending suggests that the Salaryman and his wife choose to unleash their grief on the world, nurturing their own love even as they injure others.
The final movie, Bullet Man, is a bit different. It's in faltering English, which does it a bit of a disservice (it's awfully cheesy), and has a bit too much plot. As the end of the trilogy, however, I think it works perfectly. It provides a compassionate ending to the Salaryman and the Guy (the two characters that always return to their core dynamic), suggesting that we don't have to be consumed by our own emotions as represented by grotesque metal forms.