She is supposed to have offed herself with an Asp. A story which has many problems, it wouldn't be easy getting a Snake past the guards and a snake may not necessarily be provoked to bit on command and might not deliver enough venom for a fatal dose anyway. Especially for 3 women.
Several ancient writers claim in the alternate that she used a poisoned hairpin. Which of course adds to the issue of whether the guards knew their jobs at all, surely a pin would have been discovered?
I could almost believe Octavian was behind it, whether directly or by allowing it to happen and the later would explain the otherwise incomprehensible incompetence of Cleopatras gaolers, who as far as I know, escape unpunished. But, Octavian had earlier taken great care to try and ensure Cleopatra remained alive, including essentially blackmailing her to eat by threatening to kill her children. He wanted her to be paraded in his Triumph*.
I know a former FBI profiler wrote a book claiming she was murdered, I don’t know how well regarded that book is.
*Although I have read speculation that he wasn’t planning to do so, since the Roman populace was fickle and had on occasion shown symphaty for the paradees, as had happened earlier with Cleopatras sister Arsinoe.
Unfortunately, the mystery surrounding Cleopatra's death will likely never be fully laid to rest. At this point, I feel pretty confident in saying that any speculation as to the exact circumstances other death are just that: speculation. With very little contemporary evidence, historians are left to play a game of telephone with ancient texts that paraphrase even older books.
Here are some older answers I wrote which might be helpful (but they don't really contain any new insights that you probably haven't already encountered in your research). This one about the circumstances of Cleopatra's suicide, and the (un)-likelihood of using asp venom and this one about Cleopatra's motivations for committing suicide, and the context of her death.