Why First World War aircraft lacked ship-killing bombs

Did First World War aircraft have bombs capable of killing a capital ship?

No. They could damage upperworks, but aircraft and bomb technology had not yet produced a reliable ship-killing weapon.

First World War aircraft could carry bombs that damaged a warship's upperworks, but they did not yet have a weapon capable of reliably destroying a capital ship. A decisive attack required both an aircraft able to lift a much heavier load and a suitable bomb. Neither side of that weapon system had matured by 1918. This marks the difference between early aerial harassment of a ship and the later ability to deliver heavy anti-ship ordnance.