The truthful answer to this question is that no-one is ever going to know. It's a given that Sid and Nancy were both so stoned at the time of her death that iuts perfectly possible that the stabbing was an accident, and neither of them realised how serious it was - heroin is an extremely effective anaesthetic - and Nancy may have been oblivious to her fatal wound, however she aquired it. The only people who might have an inkling about motives and circumstances are Sid and Nancy themselves, and they are both dead of course. In the nature of rock'n'roll mythologising, the rumours go round and around - the truth remains a secret.