Dovid Hamelech was not held accountable for Uriya’s actual death – since he was a mored b’malchus – but rather for his neglect to process his case in Sanhedrin.  But without a verdict from Sanhedrin it would seem that he is considered completely innocent, just as any other capital punishment transgressor who is considered halachically innocent until the Beis Din actually issues a ruling on his guilt (see Tosfos Makkos 5a D”H V’chen, and Gilyon Hashas).