Chances of dying during BBL?

Fat embolus is a very serious and known complication of fat transfer (which has been recently spotlighted in the media). While it is true, injection of donor fat into the infra-muscular plane (within the muscle) increases fat viability this does come with a significant increase in risk. There are large vessels deep to and within the muscle which can be inadvertently injected while grafting leading to a fat embolus. The consequences of such a complication are very serious and life threatening.
At the recent American Society of Plastic Surgeons Hot Topics, data from AAAASF (ambulatory surgical certifying body) was presented, suggesting that BBL/gluteal lipoaugmentation demonstrates a dramatically higher risk profile than other surgeries. The Aesthetic Surgery and Educational and Research Fund is convening a task force as well. The hope is that we will have more concrete guidelines for addressing these very serious complications.
That being said, the safest way to avoid fat emboli is to stay in the subcutaneous plane (at the expense of fat viability)and avoid the deeper muscular plane. If grafting of the deep plane is planned, surgeons should consider blunt cannulas, aspiration prior to injection. Using a supra-gluteal approach to minimize the risk of inadvertent intravascular injection has also been proposed.
With regards to your specific question, if fat is being injected there is always a risk of embolic events and thus subsequent death. As I described above, there are ways to minimize this risk. The quoted percentages will vary, but according to AAAA (certifying body for office operating rooms) there is a legitimate reason for concern when it comes to large volume fat transfer to the buttocks. The incidence of death post BBL is quoted at 1:3000-6000 and the incidence of serious pulmonary compromise (pulmonary cripple) is 1:1100. By comparison the overall incidence of death in a AAAA facility is 1:55,000. 
In summary, the procedure can be safely performed, however, they should be performed by a board certified (ABPS) plastic surgeon with experience. These risks should be discussed as well as the strategies being employed to avoid them.

While a Brazilian Butt Lift is generally a safe procedure it has the same risks as liposuction with the added risk of fat necrosis.

It is possible get a fatal pulmonary embolus or a fat embolus.  It is also possible to have lidocaine toxicity or puncture an internal organ.  The risk of all of these is much less than 1%.