It could be House Malfoy and Friends, but if so, we would still need some reason why Quirrell would not have told McGonagall that the troll was likely after Hermione. If Quirrell did want Hermione dead, he would most likely have kept any other potential murderers away from her so that her death could occur at a time and in a manner that would suit him best. So the most probable explanation is that it was, in fact, Quirrell.
McGonagall is savvy enough to know that the current defense professor is at fault for something like this; Quirrell pushed his luck in manipulating her into taking the actions she did, including providing him a single point of false-memory charm to an alibi for whatever he needs to do (including stealing a time-turner, meeting his future self, confirming that everything appears to will have gone well, and then going back to set up the scenario in the first place).
ETA: And even if it wasn’t Malfoy & co, it is believable that they would think it was one of them.
It’s also possible/likely that the Malfoy household is either involved, or believes that one of their allies is involved, in a revenge killing.
It could be House Malfoy and Friends, but if so, we would still need some reason why Quirrell would not have told McGonagall that the troll was likely after Hermione. If Quirrell did want Hermione dead, he would most likely have kept any other potential murderers away from her so that her death could occur at a time and in a manner that would suit him best. So the most probable explanation is that it was, in fact, Quirrell.
McGonagall is savvy enough to know that the current defense professor is at fault for something like this; Quirrell pushed his luck in manipulating her into taking the actions she did, including providing him a single point of false-memory charm to an alibi for whatever he needs to do (including stealing a time-turner, meeting his future self, confirming that everything appears to will have gone well, and then going back to set up the scenario in the first place).
ETA: And even if it wasn’t Malfoy & co, it is believable that they would think it was one of them.