Daf 18b

On Daf 18b the gemara teaches that the death of Tzaddikim is as great as the burning of the Beis Hamikdash.

The gemara in Bava Basra 75a teaches that Hashem decreed death upon Adam HaRishon because He foresaw that Chiram the king of Tzur was destined to arise and make himself a god. And it says in the Midrash that the Beis Hamikdash was destroyed because Chiram the king of Tzur sent cedars for its construction.

According to these two teachings we can explain our gemara - that just as the burning of the Beis Hamikdash was because of Chiram the king of Tzur, so too was the death of Adam HaRishon because of him, and thus the death of a Tzaddik is considered equal to the burning of the Beis Hamikdash.