Prigozhin is the head of the mercenary army that when they feel someone has betrayed them, clingfilms them to a post or block and crushes their head with a sledgehammer. Happened several times at the least.
When they took Soledar, a salt-mining town near to Bakhmut, they found a cellar with 300 - 400 people including children. The orders were to make sure no one got out alive, and they did.
Wagner is a terrifying organisation known for either arranging coups or preventing coups in Africa, depending on who pays them the most.
Prigozhin used to be loyal to Putin but has been increasingly critical of the MoD and in the mid-spring started to criticise Putin himself indirectly and then more directly.
It's been claimed he has been planning this for a few months.
There is no one to block the Wagnerians heading towards Moscow unless there is an army airlifted in. 5 planes landed in Belorus, presumably to carry belorussan or perhaps Russian soldiers back (depending on who is available). Other than this, there are almost no soldiers in Moscow so there's a good chance Prigozhin will make there.
There are reputedly negotiations going on between Prig and Putin at the moment. However, Putin is looking extremely weak.
Either way, Russia's in for a period of worse instability. If Putin survives, there's going to be purges of pro-Prig people; if Prig gets into power, there'll be purges the other way. Both leaders are taking Russia into a more militaristic direction, but Russia is also becoming weaker.
For Ukraine, I don't know how much difference this will make but it may fuck up already low morale in some Russian soldiers and it will probably make supplying them harder. Right now I should think the Ukrainian leadership are watching with hanging jaws, popcorn and a lot of fast planning.