Crooks had the remote detonator, too. The cop may have caused Crooks to just shoot and forgo whatever he was planning with the explosives in the backpack. Or maybe the cop’s discovery of him wasn’t bothering him at all.
Maybe he intended to set them off right when making his shot or just after, in order to disappear into the resulting chaos, but was -at last- fatally hit before he could do so.
Crooks lived at home. What, exactly, did his parents think he was up to?