Twenty-one years have passed since the death of Kurt Cobain. What can be said that hasn't already been explored by earlier documentaries (Cobain: About a Son, Kurt and Courtney) and various anniversary tributes?

Plenty, apparently. In 2002 documentarian Brett Morgen (working with co-director Nanette Burstein) found nifty ways to translate producer Robert Evans's autobiography The Kid Stays in the Picture into film. Here he has – to use his phrase – "been handed the keys to the kingdom" by Cobain's widow Courtney Love, who gave him unprecedented access to the Nirvana frontman's notebooks and the couple's sassy, hilarious, erotically charged home movies.

Most of the material has never been seen before. What emerges is a staggeringly brilliant portrait of a staggeringly brilliant artist. Using animations to both dramatise audio recordings and to bring Kurt's extensive catalogues of doodles to life, Montage of Heck has a wonderful handmade feel, not unlike Jonathan Caouette's wonderful 2003 auto-doc, Tarnation.

Interviews with Love, Kurt's parents and bassist Krist Novoselic are patient and respectful. Where other films – notably Kurt and Courtney – have sought to point fingers, Montage allows an image of a troubled, funny, contradictory human being to emerge. Whatever the failings of the people around him throughout his short life, it's clear nobody could have healed his damaged psyche.

It's equally clear that often the lesser things – notably, the legal actions, including against Lynn Hirschberg's Vanity Fair piece alleging Love was shooting heroin throughout her pregnancy – were damaging for a man known for sensitivity and a decidedly feminist sensibility.

One might reasonably expect a film that credits Frances Bean, Kurt’s daughter, as an executive producer, to revel in hagiography. But that’s not how the Cobain-Love dynasty rolls.

Instead we have a film that, despite its two-hours-plus run time, echoes its subject with a disconcertingly abrupt ending.