A spartan can be killed by a lot of things in the halo universe which is why they are nearly all dead. Mc was very very lucky in all his dealings infact lucky to be alive. Realistically spartans cant kill a forerunner or any of their constructs or ships a precursor and its star roads a covenant ship or a unsc ship ancient humans of their ships evolved flood or normal flood and their ships an overwhelming suply of enemys. A whole load of things can realisticly kill a spartan.