Recently acquired data disproves this theory, Pink Diamond committed suicide to martyr herself to Homeworld. We appreciate the attempt.
That Pink Diamond willingly shattered herself as a means of benefitting Homeworld and the Diamond Authority.
Many have theorized that Pink Diamond was on over by Rose Quartz and planned to give Earth its independence (possible even breaking away from the Diamond Authority all-together), but not this theory. This theory believes that she was still very much aligned with Homeworld's interests. Rose Quartz's philosophy of "radical freedom" is a dangerous thing, Rose Quartz herself is a very charismatic gem, having accumulated an army and resources so diverse that they were able to fend off the forces of Homeworld for hundreds, if not thousands of years.
It is clear that all Rose wanted was for Earth's independence, as well as the freedom of the gems and humans who dwell there. If Rose Quartz could spread her ideas off-planet, it could threaten the power structure that Homeworld has established and maintained for eons, the Diamonds losing their power and throwing Gem society into mass chaos in civil war. It is made clear that most of gem society believe that Earth was destroyed with the rebellion along with it, as mentioned by the off-colors.
So what does one do? Make the rebels the bad guys again. How? Show that they are no better.
Pink Diamond probably came to the realization that she simply could not overpower Rose. Seeing that she and her society could lose the battle of ideas, Pink Diamond made herself into a martyr. The instant Pink Diamond was shattered, Rose Quartz and her cause would be corrupted in the eyes of all of Homeworld, all of the Diamonds, not just that of Pink Diamond. Those who allied with Rose and her beliefs were suddenly not just swimming against the tide of history anymore. They were a real threat to the way of life that they have grown to accept. This unites Homeworld against an enemy, their jingoist mindsets reinvigorated.
Side-note, it is equally likely that Yellow Diamond knew about what Pink Diamond planned on doing and could have been involved in it. Whether it was because she was unable to stop her or maybe because Pink asked Yellow to help in an assisted suicide. Because of this, Yellow Diamond's attitude could be a by-product of the grief of keeping what happened a secret, as well as guilt for her actions (or inactions).