Which one would be the correct form for this context?
This situation is a Hypothetical and takes place in future.
He would be killed when he arrives in London.
He would be killed when he arrived in London.
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It should be He would be killed when he arrives in London.
Or He will be killed when he arrives in London.
Would is a conditional modal verb. It would be hot when he leaves the plain in Texas after his trip tomorrow. The condition is his arrival then. The tense of what happens then belongs to that as if it were now.
If this is last years plan that failed you would be telling it as "Our plan was that He would be killed when he arrived in London." There the tense belongs to back then.