This is not a standard expression, my guess would be the speaker is suggesting that the criminal may already have been chopped to bits (dismembered or otherwise murdered brutally), in the same way that coleslaw is tiny pieces of sliced up vegetables.

Seems to me that they are saying he is dead, murdered, executed, depends on what and who they were talking about, The second officer says we will know if he is. However there is now apparently a new social media slang term "coleslaw" that means someone's partner is cheating on them. "coleslaw" being a side-dish. an ancient slang term for a partners illegal already married partner is "a bit on the side".

I'm a native US speaker and it makes no sense to me. It's not a standard expression. Maybe it refers to something they were talking about earlier.