Perhaps Spock's logic told him that the needs of the many outweighed the needs of the few, and came up with some Jack Pack style situation.
Spock uses red matter or trilthium or similar to stealthily trigger a supernova of Romulus - with a few years notice, giving enough time to evacuate the planet -- this would lead to aversion of a conflict that's brewing (as predicted by Sloan), and reunification of the Romulan and Vulcan people, with minimum if any loss of life. The timescale is short enough that the Romulans need help from the Federation to evacuate on time, further cementing the bond. Perfectly logical - he'd been working for over 100 years to reunify Romulus and Vulcans, and this was the last chance.
The attack on Mars was not predicted, meaning the Federation pull out, which didn't leave Spock with much time to come up with a new plan. He thinks he can still accomplish his goals (reunification and peace) by sacrificing himself so that the Romulans think that the Vulcans at least, were willing to go all out to try to save them.
Spock doesn't want this evil plan filtering through the Pine-Kirk though. Kirk being young, rash, and undisciplined and completely new to mind melds, easilly falls for whatever the old experienced Spock fabricates.