Why did Billy flee after identifying the supplier?

Incredible film and epic score. But i could never understand why Billy fleed when he pointed out the taxi driver who supplied him? If he had stayed put, surely the police have let him go for cooperating?

They said they would, so i don’t get why he ran. Panic I assume, and I got the feeling Billy was wreckless and didn’t think things through..

Then again, we wouldn’t have a great film.

Thoughts?