Can I use Forge of the fluft to equip blade of the ruined king without killing a unit?

Yes — Forge of the Fluft's granted ability just Attaches the Equipment; it never has you pay the Equip cost, so the "Kill a friendly unit" clause on Blade of the Ruined King is never invoked.

Forge of the Fluft gives friendly legends "[E]: Attach an Equipment you control to a unit you control." That's a bare Attach action (Rule 434), not the Equip ability — no cost from the Equipment's rules text is referenced or paid. The kill requirement lives entirely inside Blade of the Ruined King's [Equip] cost (Rule 818.1.c.3), so it only matters if you activate the normal Equip ability. You still need to have played the Blade from hand normally (3 energy + 1 Order power) first; Forge just moves it, and it can even pull it off a unit it's already attached to since the granted ability lives on the battlefield rather than the now-inactive gear (Rules 718.2, 818.5.a).