Chauvin and Floyd knew each other. They worked together as bouncers on the same security team at a nightclub for several years. It's safe to say they've met before. Personal animosity is likely the motivation for the killing. When a guy who kills another guy that they know, it's usually over money or a woman.
That's the key to the whole case for the prosecution. To show the personal motivation and the animosity between the two men.
And this is why the rush to indict on the part the Minnesota AG was so stupid. No one is likely to be ready to talk now. Keeping it in the "investigation phase, where different people who knew the guys would have made it easier from people to open up.
Further, Keith Ellison et al , seem committed in advance to the theory that this was a "Hate Crime". Chauvin may well have hated Floyd, but its unlikely that it was due to Floyd's racial identity. A lot more likely that Chauvin thought that Floyd had poisoned his dog and fucked his wife, or vice versa, and that was the his motive.